The deal with iodine tablets is that the body only absorbs as much iodine as it needs, and everything else passes through. So if you take a tablet of safe iodine to max out your body's supply, when you are exposed to radioactive iodine particles, your body won't need any and won't absorb them. I think iodine collects in your thyroids which are especially sensitive to cancer, so keeping radiation away from them is a good thing.

As for the reactor 4 pond being dry, the BBC article I linked to seemed to be saying that adding water to the well is a catch-22, because adding it could SPARK the nuclear chain reaction by slowing down the radiation and moderating it. With the well being dry, the radiation moves so fast it doesn't get absorbed by the rest of the nuclear fuel, so the chain reaction can't start.