[mhandley]

The main issue as I understand it is that you lose a lot of energy to heat if you just pump air straight in. To be efficient you need to chill the compressed air down to water temperature and store the heat before you pump the air underwater. Then add the heat back to the air when you release it from the balloon. This is all doable, but with all those heat pumps it's not quite as simple/cheap as it might seem.