Because it is fairly good conductor of heat and readily available?
That is a very good answer. Another reason is that water does not ‘retain’ radiation. Particles in the water might, but the water itself does not.
Well, that is good to know about the water itself, but then I wonder about plankton, fish, kelp, etc....
Ah yes... the food chain. Of that you are correct in having some concern. I was only trying to clarify that testing the water itself would be pointless.
As I have previously said, now you can take all the info from Freepers Input on this thread, evaluate the various responses, and resolve your problem.
Don't let any negative comments distract you.