“New Scientist” article on the evolution of whales and why they didn’t learn to breathe water…
…basically it’s too much hard work for not enough oxygen. Whales, as mammals, need a good oxygen supply and there just isn’t enough in sea water to get at without the whale getting exhausted. Moving all that water in and out of one’s lungs/gills is very hard work – a mammal would need to breathe kilograms of sea water at a time rather than the mere grams of air it does breathe.
Often people wonder: why don’t we colonize the sea instead of space since it is so much closer?
But how close is it really? Beyond a few metres the pressure rises to levels that challenge our freedom to move – you can’t ascend too rapidly without risking the “bends”. Plus there’s very little oxygen, little light and very little else. In space pressure is never a problem and light is everywhere. In the sea any kind of heat processing suffers from the heat-sapping presence of water, but in space one can vapourise metals and silicates via concentrated sunlight.
So I have a few reservations about the whole “colonise the sea instead of space” idea. As NASA has long realised the two require similar and yet dissimilar technologies – it uses underwater laboratories as training environments for its astronauts, but isn’t planning on colonising the great oceanic deserts anytime soon.
Kurt9’s comment makes another good point…
The ocean is a remarkably hostile environment from a structural engineering standpoint. Seawater is corrosive and you have 50 meter rogue waves to deal with. You also have to deal with typhoons if you are not within 5 degrees latitude of the equator. Space is actually a more benign environment for structures, but is very expensive to access as of yet.