Why aren't the reactors and cooling tanks of nuclear power plants built below water level?

My friend asked me this today and I couldn’t come up with a good reason. His point was that if there ever was an emergency, they could flood the reactors and cooling tanks with gravity, instead of relying on pumps that could fail in a disaster like what happened in Japan. I told him someone probably thought of this, but there are probably good reasons why they don’t design them this way. So why don’t they build them below the water level?