Umm...the oceans aren't going to boil away, not unless we nuke the planet or the sun explodes.

Cold fronts may lower it 15 degrees, but that is the surface temp, underneath it takes a lot longer to cool that much. Also, what about the species that can't swim away, like coral? What happens to that? How about when the fish swim back and you release more cold water again? Then they swim away, then swim back. Then they probably do it again, and again, and again. That is until they are so stressed out from the temperature changes that they simply give up and die.

One more thing, your idea won't stop global warming. The energy is simply going somewhere else on the globe. It can't go away, goes against the laws of thermodynamics. The only way to stop global warming(if it even exsists, thats still debatable) is to remove heat from the earth. Sounds like another project for you.