It's a fusion reactor, so how do you get the water into the part with the hot stuff? That's all inside a magnetic field, I think. Also, you have to consider the neutron radiation's effect on the pipes that carry the water into the core, it's going to activate several metals in there, and cause weakening of the pipe wall, which will necessitate inspections and replacements.

It's a way harder engineering problem than you're letting on. Your comment is like a software user saying "How hard could it be just to add feature X?".