could we colonise the sea floors using "open air" tanks filled with liquid breathing fluids like a perfluorocarbon.? because its such liquids are denser than water and insoluable, all you need to do is make some walls and fill the bottom with a few meters of liquid breathing fluid. Home habs would obviously be pressurised and air filled but outdoor activities wouldn't be constrained by the large pressures and the need for massive expensive suits. Engineered plants could supply the oxygen, theyd get water from the water trapped between dirt grains, and the co2 and oxygen from the breathing fluid like us. it would be a closed circuit open "air" tank habitat.