Water does indeed reduce in volume under pressure; not as much as nitrogen or hydrogen at the same pressures, but non-trivially all the same. If you could prevent that, the sea might be several metres deeper than at present.

More realistically, by raising its temperature until it obeys Boyle's and Charles's laws, we could make its behaviour correspond to normal gases at corresponding temperatures, in which case the volume of the sea might be taken to be several hundred times its current value, much more than that of our current atmosphere.

Have fun calculating it!