Well, the old fashioned way is to box in the area in question and then go to town with gravel/sand. This route is actually remarkably fast for getting rid of water, it's just that you need to dig everything back up. Some sort of automated mining could cure that, though.
If you're on 1.5, pumps should still work in theory. With an REC next to it, I've seen pumps obliterate rivers, so I assume they could do the same for an ocean. If you're on 1.6, though, pumps were changed to not drain large pools of water and just treat them like an infinite source.