Oceans may be rare in the inner Solar System—Mars and Mercury are too small for oceans while Mercury and Venus are too hot—but if we consider that water is composed of hydrogen (the most common element in the universe) and oxygen (the third most common element), it seems likely that water would be pretty common too. Indeed, if we look at the worlds out beyond the Solar System’s frost line, we note that there are likely to be oceans within Europa, Enceladus, Ganymede, Ceres, Pluto, and other small worlds.
As for exoplanets (which we have been discovering at a surprising rate, of late) … well, some of them must have oceans, or be covered with oceans, as well. SF authors, even before the exoplanet boom, have long been imagining water worlds. Here are a few books about ocean planets.
The Blue World by Jack Vance (1966)
Generations ago, human refugees found a world with no land. There were only floating plants—Floats—on which to make their homes. Which they did. They then found that their settlements had few defenses against native predators. The humans reluctantly made a pact with a giant intelligent predator named King Kragen, providing Kragen with food in exchange for protection.
The human Sklar Hast protests this arrangement. He doesn’t get much of a hearing; the powers-that-be are satisfied with things as they are. If Sklar is so unhappy, let him go find a new life, as an exile in an ocean wilderness where humans are nothing but food.
A Door Into Ocean by Joan Slonczewski (1986)
The Sharers of ocean moon Shora are parthenogenic pacifists and highly advanced in biological sciences. They have used their expertise to adapt themselves to their moon. Into this idyll come the imperialist Valedon, who regard the aquatic Sharers as less than human. Valedon has plans to monetize Shora and its resources. The conquerors expect no resistance from the pacifist Sharers. What can they do, if they’ve renounced violence?
Noise by Hal Clement (2003)
The water world Kainui is inhospitable: there’s no dry land, huge thunderstorms are frequent, the atmosphere is anoxic, and the lifeless acidic oceans are almost three thousand kilometers deep. Perhaps it could be terraformed…but who would bother? Turns out that some humans are willing to live there even without terraforming.
Several centuries ago, Polynesians settled the planet armed with pseudolife, a form of advanced nanotechnology. They have thriven and multiplied. New languages have evolved.
Terran linguist Mike Hoani arrives to study these languages. He’s received with a notable lack of interest. The locals aren’t hostile, but they aren’t putting themselves out to be helpful. There are no free lunches on Kainui; if Mike wants to study Kainui linguistics, he will have to find a paying position where he can learn languages as he works. But first he has to manage a crucial skill: surviving on an unfamiliar world.
Aria by Kozue Amano (2001–2008)
By the 24th century, Mars has been transformed from a nearly airless desert world to a hospitable ocean world. It has been renamed “Aqua.” The largest city on Aqua is Neo-Venezia, whose architecture and narrow canals deliberately recall those of Earth’s Venice.
Akari Mizunashi dreams of becoming an Undine, one of the young women who pole gondolas through Neo-Venezia’s canals. This requires hard work and persistence. As readers follow her progress, they learn more about Neo-Venezia and the world around it. Aria is notable for appealing characters and lush artwork.
Arkfall by Carolyn Ives Gilman (2008)
The frigid moon Ben lacks native life…but it now has humans. The human settlers have dedicated themselves to the Great Work, a vast project which will draw on the geothermal power of the Great Cleft to turn one small corner of Ben into a living sea. This is the work of lifetimes, one that demands dedication, cooperation, and self-sacrifice from the local humans.
The Great Cleft is a dangerous place in which to live and work. An unanticipated eruption sends the free-floating habitat of Divernon out of familiar, protected waters and into the World Ocean. The tiny habitat is self-contained and self-sufficient; it should be able to sustain the three humans trapped inside as long as they can stand each other. Note the word “should.” Ben has surprises in store for the inhabitants of Divernon.
No doubt tales of alien oceans are as common as the alien oceans will eventually prove to be in real life. Feel free to mention the ones I’ve overlooked in the comments below.
In the words of Wikipedia editor TexasAndroid, prolific book reviewer and perennial Darwin Award nominee James Davis Nicoll is of “questionable notability.” His work has appeared in Publishers Weekly and Romantic Times as well as on his own websites, James Nicoll Reviews and Young People Read Old SFF (where he is assisted by editor Karen Lofstrom and web person Adrienne L. Travis). He is a four-time finalist for the Best Fan Writer Hugo Award and is surprisingly flammable.