Ocean colonization is a process of populating the Earth's oceans(or terraformed world's oceans). This would prevent overpopulation from occuring. It would be successful by putting biosphere habitats in the middle of the ocean or on the ocean floor.
Not just Earth, a terraformed world, or a settled earth like exoplanet could sustain ocean colonization but icy dwarf planets and moons could have subsurface oceans. Some fresh water, others salt water. Building cities floating in the oceans of a icy moon is possible. Although, chances of finding harmful extraterrestrial life forms are very high. It could be complex plant/animal like organisms. No matter if they are intellegent or unintellegent, they can still be dangerous. An intellegence could be dangerous for their advanced technology and unintellegent aliens could be dangerous for their instincts.
It would be possible to use a gas that cannot dissolve in blood instead of nitrogen to prevent bends.