I know this is planned: building bases underwater and then draining them with a pump, but since underwater bases would be significantly more difficult to raid than a counterpart on land, there should be another factor to balance it out: oxygen.
(Note that all values provided here are simply for reference only)
For the sake of something to relate to, let’s say there is a player in a 1x1 underwater, and the 1x1 is completely enclosed, airtight, and pumped dry. The 1x1 can hold a maximum of, say, 100 units of oxygen. The player would use up the oxygen in the room at a rate of, say, five units per second. However, by draining all the water in the room, there is nothing filling the room except what can basically be considered a vacuum.
In order to provide oxygen to breathe, the player can build, craft, and find various items and machinery to provide oxygen into the base.
Plants
This one should be a no-brainer. They provide oxygen at a rate of, say, 1 unit per second, meaning 5 plants should be enough to sustain 1 player in a 1x1 base.
Chlorate Candles
Burning these candles releases oxygen at a fixed rate of say, 10 units per second for 100 seconds, and they can be found in space-related locations.
Electrolyzer
Requires electricity to function, and separates water into hydrogen and oxygen. Releases oxygen at a rate of say, 10 units per second.