also, it's pretty illogical talking about "ionizing water" because water IS ions. anything that dissolves in water is ionized by the water.
i think people have a mixed up perception that there is some connection with nuclear decay radiation and microwaves. there is not. most of what comes out of radiactive decay is mostly fragments of nuclei, alpha and beta particles, protons, neutrons. these cause cancer because they readily break anything they hit, especially DNA and proteins.