Why the race to colonize space if we can colonize the oceans?

Instead of building huge space habitats, why don't we bio-form (genetically alter ourselves) so that we can live underwater? Underwater, we could pretty much "fly" all the time, 2/3 of the earth's surface is water, and we could have layer upon layer of habitat there--so there'd be so much more space for us. Sure, the engineering would be tough--probably couldn't even call the finished product "human" anymore--but still, I don't get why people don't go for it.