I can't speak to the efficacy of water to prevent magnetrons from overheating. But yes, you can safely "cook" a microwaveable bowl of water for as long as you have water.

I can't speak to the efficacy of water to prevent magnetrons from overheating. But yes, you can safely "cook" a microwaveable bowl of water for as long as you have water.

See the link on Stephie's answer about overheating if you have to touch the bowl or water before it has the opportunity to cool.