Earth, with oceanic water covering 71% of its surface, is the only known planet with liquid water on its surface and is certainly the only one in the Solar System. Oceans and water may be common in other star systems and/or on their planets and other orbiting celestial bodies: for example, water vapour has recently been found at the right spot in a protoplanetary disc.
Large bodies of water and extensive water systems were once thought to cover the Moon, Venus, and Mars. With increased telescopic resolution and enhanced observation techniques in modern times, these were ultimately disproven; however, the presence of water on Mars in the distant past remains a topic of contemporary debate.
Lunar maria are vast basaltic plains on the Moon that were thought to be bodies of water by early astronomers, who referred to them as "seas".
Before space probes were landed, the idea of oceans on Venus was credible science. But it was discovered to be much too hot.
Mars was never supposed to have oceans. Its dryness was long recognised, and gave credibility to the spurious Martian canals.
Assuming that the Giant impact hypothesis is correct, there were never real seas or oceans on the moon. Astronomers believe that Venus had liquid water and perhaps oceans in its very early history. Given that Venus has been completely resurfaced by its own active geology, the idea of a primeval ocean is hard to test. Rock samples may one day give the answer.
It was once thought that Mars might have dried up from something more Earth-like. The initial discovery of a cratered surface made this seem unlikely, but further evidence has changed this view. Liquid water may have existed on the surface of Mars in the distant past, and several basins on Mars have been proposed as dry sea beds.  The largest is Vastitas Borealis; others include Hellas Planitia and Argyre Planitia.
There is currently much debate over whether Mars once had an ocean of water in its northern hemisphere, and over what happened to it if it did. Recent findings by the Mars Exploration Rover mission indicate it had some long-term standing water in at least one location, but its extent is not known.
It is thought that liquid water may exist in the Martian subsurface. Research suggests that in the past there was liquid water flowing on the surface, creating large areas similar to Earth's oceans. However, the question remains as to where the water has gone. There are a number of direct and indirect proofs of water's presence either on or under the surface, e.g. stream beds, polar caps, spectroscopic measurement, eroded craters or minerals directly connected to the existence of liquid water (such as Goethite). In an article in the Journal of Geophysical Research, scientists studied Lake Vostok in Antarctica and discovered that it may have implications for liquid water still being on Mars. Through their research, scientists came to the conclusion that if Lake Vostok existed before the perennial glaciation began, that it is likely that the lake did not freeze all the way to the bottom. Due to this hypothesis, scientists say that if water had existed before the polar ice caps on Mars, it is likely that there is still liquid water below the ice caps that may even contain evidence of life.
Subsurface oceans have been postulated for most of the icy moons of the Outer planets, which are covered by a layer of water ice. In some cases it is thought that an ocean layer may have been present in the past, but has since cooled into ice.
Liquid water is thought to be present under the surface of several natural satellites, particularly the Galilean moons of Jupiter, such as Europa (liquid water underneath its icy surface due to tidal heating), and, with less certainty, Callisto and Ganymede.
Geysers have been found on Enceladus. These contain water vapour and may mean liquid water deeper down. The water is either heated tidally, or geothermally. It is known that Enceladus has liquid water as there are active cryovolcanic mountains around its southern pole. It could also be just ice. In June 2009, evidence was put forward for salty subterranean oceans.
It was believed after the Voyager observations that Titan might have seas or oceans of liquid hydrocarbons. The Cassini-Huygens space mission initially discovered only what appeared to be dry lakebeds and empty river channels, suggesting that Titan had lost what surface liquids it might have had. A more recent fly-by of Titan made by Cassini has produced radar images that strongly suggest hydrocarbon lakes near the polar regions where it is colder. Titan is also thought likely to have a subterranean water ocean under the mix of ice and hydrocarbons that forms its outer crust.
Neptune's moon Triton may have once had internal oceans that have now frozen. This could also be true of other icy moons.
On Friday, November 13, 2009, NASA announced that it had found a "significant amount of water" on Earth's Moon. This water was in ice form; however the discovery lends credence to the theory of liquid water on distant planets.
Jupiter possesses a gaseous layer where, because of the Earthlike temperature and pressure, droplets may condense from the water vapor.
Uranus and Neptune may possess large oceans of hot, highly compressed, supercritical water under their thick atmospheres, though their internal structure is not well understood at this time. It is agreed that they are different from the gas giants Jupiter and Saturn—some astronomers would class them separately as 'ice giants'.
The dwarf planet Ceres is believed to contain large amounts of water-ice, and might possess a tenuous atmosphere. It is too cold for liquid water, but an ocean of water plus ammonia has been suggested. More information will be available in 2015, when the Dawn Mission visits it.
The Solar System may not be typical. Most of the 200+ detected star systems look very different from ours, though there is probably a bias arising from the detection methods. The hope is for Earth-sized planets in the habitable zone, (which is also sometimes called the Goldilocks zone). Planets with oceans could include Earth-sized moons of giant planets, though it remains speculative whether such 'moons' really exist. The Kepler telescope might be sensitive enough to detect them.
55 Cancri f is a large planet orbiting in the habitable zone of the star 55 Cancri A. Its composition is unknown but it is believed to be a gas giant. If it has rocky moons, these could have liquid water.
There is also a gap in the orbits of that system's five (known) planets which might contain something more Earth-like. If it exists, it cannot be detected by present methods, though these are constantly being improved.
AA Tauri is a young star, less than a million years old and a typical example of a young star with a protoplanetary disk. Astronomers have recently found the spectral signatures of water vapor, plus three simple organic molecules - hydrogen cyanide, acetylene and carbon dioxide. Solid bodies condensing from the disk should have liquid water, if they are the right distance from the star.
The new world orbits very close to its parent star, such that its surface is a scolding [sic] 1,000–1,500 degrees Celsius. Depending on the density of the planet, temperatures like that suggest the surface could be covered in molten lava or enshrouded in a humid cloud of water vapour. The planet could even be made up of water and rock in almost equal amounts.
COROT-7b could represent an ‘ocean planet’, a kind of planet whose existence has, so far, never been proved. In theory, such planets would initially be covered partially in ice and they would later drift towards their parent star, causing the ice to melt and flood the planet with liquid. 
COROT-9b has been called a temperate exoplanet, the size of Jupiter but a similar distance as Mercury is from our Sun. There are other cases known, but this planet can be studied in detail because it transits its star. And it may have liquid water:
“Like our own giant planets, Jupiter and Saturn, the planet is mostly made of hydrogen and helium,” says team member Tristan Guillot, “and it may contain up to 20 Earth masses of other elements, including water and rock at high temperatures and pressures.”
The planet passes in front of its host star once every 95 days and takes eight hours to make the transit, offering astronomers the chance to glean bountiful information on the nature of the planet. The team report that CoRoT-9b has an unusually temperate climate, with the temperature of its gassy surface ranging from -20 degrees Celsius to 160 degrees, and with minimal day-to-night variations. 
A white dwarf star called GD 362 may have swallowed a small exoplanet full of water. As reported in Astronomy Now:
The white dwarf in question… located 150 light years away in the constellation Hercules, has been in the news ever since odd amounts of heavy elements were discovered in its atmosphere in 2004, followed a year later by the detection of a ring of broken-up asteroids encircling it. It is easier to understand this ring when you consider what a white dwarf is: the final evolutionary step of a Sun-like star, after its hydrogen fuel in its core has run out and it has swollen into a red giant, before ejecting its outer layers to, leaving behind its helium-rich core in the shape of a white dwarf. During the red giant phase, the star engulfs and destroys its innermost planets, and disrupts the orbits of outlying bodies. So what we see today in the GD 362 system is a dead star system.
The hydrogen in question amounts a hundredth of the total mass of Earth. This could be explained if an object, at least as massive as Jupiter’s moon Callisto and possibly as massive as Mars, and containing more water (the source of the hydrogen) deep inside it than even Earth does, was ripped apart by the white dwarf’s gravity and the remnants pulled in by the dead star. However, this was unlikely to be the destruction of an Earth-like planet with oceans, says one of the researchers, Jay Farihi of the University of Leicester… “If it had surface water, it was all evaporated during the giant phase of the host star”. 
Gliese 436 b is believed to have 'hot ice'. (Hot ice is a water forced into a solid state because of intense pressure rather than freezing solid from cold.) The temperature on Gliese 436 b is not cool enough for liquid water, but if water molecules are shown to exist there, then they are likely to also be found on other planets at more suitable temperatures. As reported in Science Daily:
The detection of such a hot ice world has important consequences. It shows for the first time that planets similar to the "ice giants" Uranus and Neptune... exist at close distances from their star… Many of the planets of similar mass detected around other stars by the astronomers may therefore also be composed mainly of water. Some of them will have cooler temperatures, allowing the water on the surface to be liquid. Such planets covered by a single huge ocean have been dubbed "ocean planets" by the specialists.
Gliese 581 c, a world five times the size of the Earth, was originally reported to be the right distance from its sun for liquid water to exist on the planet's surface. Since it does not transit its sun, there is no way to know if there is any water there.
Later work suggests that Gliese 581 c would probably be too hot for liquid water. It was then suggested that Gliese 581 d might be warm enough for oceans if a greenhouse effect was operating.  Gliese 581 d is eight times the mass of the Earth and might have a thick atmosphere.
As of April 2009, Gliese 581 d looks an even better candidate. The orbital period was originally estimated at 83 days and has now been revised to 66 days. As reported in New Scientist:
A planet orbiting a red dwarf star 20 light years away could be the first known water world, entirely covered by a deep ocean…
Astronomers have now revised its orbit inwards, putting it within the 'habitable zone' where liquid water could exist on the surface". 
This was announced along with another new world, Gliese 581 e, which is twice the size of Earth but too close to the sun for liquid water.
GJ 1214 b is only the second exoplanet (after CoRoT-7b) to have an established mass and radius less than those of the giant Solar System planets. It is three times the size of Earth and about 6.5 times as massive. Its low density indicates that it is a mix of rock and water. But "though astronomers are pretty certain the water exists, they don't know its state, with speculations ranging from liquid water to water ice and an exotic state called a superfluid".. Studying its atmosphere should be possible and will tell us more. 
HD 28185 b was the first exoplanet to be detected in the habitable zone. The planet has only been detected indirectly, but is believed to be a gas giant, with no solid surface. Some scientists have argued that it could have moons large and stable enough to have oceans.
The disk around a star called HD 113766 may be forming an Earth-like planet that will have oceans in the future, and perhaps life also. It is still a very young star, unlikely to have these features for hundreds of millions of years. 
HD 209458 b may have water vapour in its atmosphere - this is currently being disputed. It is not cool enough for liquid water.
The planet orbits its host star or brown dwarf with an orbital radius similar to that of Venus. But the host is likely to be between 3,000 and 1 million times fainter than the sun, so the top of the planet's atmosphere is likely to be colder than Pluto. However, the planet is likely to maintain a massive atmosphere that would allow warmer temperatures at lower altitudes. It is even possible that interior heating by radioactive decays would be sufficient to make the surface as warm as the Earth, but theory suggests that the surface may be completely covered by a very deep ocean.