Why can’t it stay liquid?

Well, the water is necessary for for life support and needs to be sourced somehow anyway. It kind of sets a minimum crew and passenger capacity if you want to make the most use of your shielding water.

Water doesn’t have to be a liquid, but don’t actual spacecraft typically contain liquids during wall of those cases? What do you mean?

I don’t think it works that way. The water slows down the neutrons so that when and if they get to you they don’t have enough energy to hurt you. The radiation doesn’t contaminate the water any more than a microwave oven does.

Since the astronauts need water to survive, why not line the spaceship with reservoirs of it to provide the shielding? Or does water not block space radiation well enough?