In Kevin Costner’s Waterworld (the 1995 movie) the Earth’s polar ice caps have melted completely, drowning the entire planet. In reality, there isn’t enough ice for that to actually happen (thank goodness, because we’re certainly doing a number on the ice we do have), but that doesn’t mean that a waterworld isn’t possible somewhere else. Even within our own solar system, giant moons like Ganymede and Europa are thought to be mostly ocean covered by ice. Elsewhere in the galaxy, a fair number of near-Earth-sized planets have been discovered that scientists believe could be substantially made of water, including Gliese 1214b and Kepler 62e. (Exoplanets are named after their parent star, with a lower case letter signifying their position among the star’s planets—“a” being the closest. These days, stars are most often named according to the sky survey and/or telescope responsible for their discovery.) A solar system thirty-nine light years from Earth known as TRAPPIST-1 is in a very favourable position to be studied, and is thought to have four waterworlds among its seven-planet roster. One of them might be composed of as much as 50% water! (Earth is only between .5% - 1.0% water.)
How do we know all this?
It’s important to explain that scientists discover exoplanets by noting the dimming of the light as the planet crosses in front of its star. Adding careful timing measurements, they can distinguish how many planets there are in the system and their orbital speeds, and determine from there the approximate sizes and masses of the planets. If the positioning is right, they can do spectrographic analysis of the star’s light passing through the planet’s atmosphere, giving them some idea of the planet’s composition. All of this data is compared to what we know about rocky planets like Earth and gas giants like Neptune. Stir the numbers all together and…voilà, an artist’s rendition complete with colours and swirling clouds and….
Well, OK, let’s just say that there’s still a fair bit of speculation involved. But they’re good guesses. So it’s reasonable to assume that a fair number of planets out there in the habitable zones of their stars (warm enough for liquid water) are really wet. That could be a good thing (on Earth water is always associated with life) or a bad thing (without land, where would life forms get minerals and other nutrients? A really deep ocean would have ice covering the bottom due to pressure, preventing material from leaching out of the ground beneath.)
The science fiction writer/futurist will say, “Aha, but who knows what forms alien life can take? Before we discovered thriving colonies of life around deep-sea hydrothermal vents we thought that all Earth life ultimately depended on photosynthesis. So there!” (We SF writers can sometimes be insufferable know-it-alls.) We’d also point out that a watery planet could be an excellent source of hydrogen for spacecraft fuel, and oxygen for, you know, breathing. Plus humans are pretty good at making floating things. As long as there are some metals and hydrocarbons around, we could readily make floating colonies that would produce food by growing algae and then farming algae-eating sea life. Underwater habitats are also cool—I’ve written about them myself. Comic books and B-movies love whole underwater cities, but there have to be very strong reasons to take on that challenge (maybe mining the materials needed for the floating colonies!) Certainly, advancements in super-strong nano-materials will make those ventures more feasible. Water planets could also provide protection against hard radiation from space, asteroid strikes, or even interplanetary war. And, dare I say it, they’re the perfect setting for pirates! (Though that is wandering across the line into fantasy.)
Even with all of this potential, I’m not aware of many science fiction stories set on or under the water on planets other than Earth, maybe because our own oceans are still enough fertile territory for the imagination. You might set me straight on that. Or you might want to take that ball and run with it yourself.
Just don’t expect anybody to make a movie of your book. Kevin’s was a bomb.