Nigel - in my little homework problem, I pretended T was constant. Of course, it probably won't be...it'll drop with depth, making the density of both water and air drop as well. If they drop at the same rate, my estimate may be correctish, if they don't, who knows....

Gareth - my guess is that a very small percentage of the ocean is deeper than five miles. Only the deepest trenches at island arcs (Marianas at 7 miles, etc.) make it that deep, and they are narrow and few.

wish this map had a true depth scale