In a story I have been working on, there is a planet called Ieuclite. I want to have its surface be entirely covered in ice and have the people live in cracks and fissures. But then I thought, what if there is an ocean beneath the surface? And what if these people evolved to be able to stay underwater for very extended periods of time (yet not fully aquatic). Some other information is that this planet is technically a moon that people from another planet colonized so they could collect the water (for other reasons that go way beyond the point of this question).
I want to keep my idea as realistic as I can, and I often tell myself, "it's just fiction, no one will really care," but at the same time I really want to have science backing up most, if not everything I write.
Also, any ideas regarding the actual species living on this planet would be greatly appreciated as well.
Edit: The aliens that colonized the planet for water come from a very dry and large planet with very heavy gravity. I read in a different Answer on this site that in heavy gravity, it would be favorable to sleep in water with breathing devices, because the water could still hold up your weight and keep you afloat. So they need to harvest more water that's readily available, and they decided to colonize the moon so they could just send little pods back with the water rather than travelling back and forth.