[Toddsterpatriot]

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.

[Kevin OMalley]

"Unless it sank in 5 feet of water, no steam would reach the surface."

Yikes, that means the sea itself would be radioactive. I seem to remember lava flows from Hawaii causing steam vents at several hundred feet of depth, but you're probably right, there's a point where the steam transitions back into water. Either way, we're looking at a terrible mess.