There are radiation waves, and radioactive particles/elements (that continuously emit radiation waves). Radiation itself alters the molecular structure of the elements that it hits. If it hits the body, molecules and body cells alter or mutate and fatal damage can be done. It's complicated. Stopping radiation can actually produce a second type of radiation from the collision. If it's stopped by water or other barriers, molecules can be altered, but mostly upward to a higher isotope. With water, not much happens. What needs filtering or removal is radioactive elements or isotopes that are in something like water in a stream that you want to drink. If it's Cobalt-60 fallout, it will kill you and it doesn't take very much to do that. Ion exchange, distillation and reverse osmosis will all reduce radioactive particles in water. But then you would have a radioactive water filter or equipment. Radioactive particles actually have to be mechanically removed (washed away) and stored till their half life goes away and they aren't dangerous anymore. Water irradiated by x-rays and gamma rays isn't horribly dangerous, but organic life can't tolerate the exposure to high amounts of x-rays and gamma rays. edit: and no, iodine drops for the water won't do it. That's for killing organic bacteria and stuff. And that's the best I can explain. A nuclear physicist would slap me silly.