"You're kidding right? Mushroom cloud?"
Okay, maybe that's not an accurate description of the downside, but it saved a lot of wordiness. The downside would be a giant column of radioactive steam rising from the overheating, sinking reactor, lasting possibly several months before the nuclear reaction plays itself out. This radioactive steam could poison everything it touches. Think Chernobyl but in a giant tub of water. In some respects, a mushroom cloud would be preferable.
Unless it sank in 5 feet of water, no steam would reach the surface.