An ocean is a body of liquid, usually saltwater. There are two planets that have oceans: Kerbin and Eve. Laythe, a moon of Jool, is both the only moon with an oxygenated atmosphere and the only moon with an ocean.
Both Kerbin and Laythe have oceans of liquid water. Laythe's oceans appear to be saltier than Kerbins', as it does not freeze at 'normal' freezing temperatures (about 0* Celsius, 32* Fahrenheit). Eve, on the other hand, has oceans of a purple liquid called 'Explodium'. It appears to be rocket fuel.
The oceans on the three bodies usually are their own biome. Kerbin's oceans are the 'Water' biome; Laythe's, the Sagen and Degrasse seas, and Crater and Crescent Bays; Eve, the Explodium, Eastern and Western seas, and Akatsuki and Crater Lakes. Kerbin's Shores biome and Eve's and Laythe's shallows biome all represent the boundary between their oceans and their land.
In Kerbal Mythology
|“||Moho figures in Kerbal mythology as a fiery place with oceans of flowing lava.||”|
However, Moho does not, in fact, have oceans of lava or any other liquid.
Things to do in Oceans
As oceans are liquid, vessels can sink into them (At least until they hit the seafloor). Submarines, like this one, can be built and piloted in Kerbin's seas.