Sealed water is all you need.
"Irradiated" water and food do not themselves become radioactive. Irradiation can cause some chemical reactions, but for water they are minimal and self-restoring. For foods, some nay-sayers will tell you that "bad, stable" chemicals are formed, but IMO the health benefits of bacteria-free food outweigh the small amounts of "bad" chemicals, if any.
My suggestion? Just keep fallout from entering your food and water supplies, don't worry about any irradiation.
Quick course in radiation:
light is a form of radiation. So is heat. The radiation that causes us trouble is high-energy, higher than light or heat.
alpha radiation - a small atom nucleus, travelling at relatively low speed. Destructive capability, high, but is stopped by a sheet of paper. Only dangerous to the body when emitted inside the body.
beta radiation - an electron from an atom, travelling at high speed. Destructive capability, high, but is stopped by a sheet of plastic. Mostly only dangerous to the body when emitted inside the body.
gamma radiation (and it's kissing cousin, x-rays) - pure energy, travels at the speed of light. Stopped by thick layer(s) of lead or tungsten. Only a small percentage of gamma rays cause damage to the body, most pass through the body. Destructive capability, low but real.
This is an oversimplification, but there are courses at most science colleges if you want to take it further.
You keep food and water free of fallout so that alpha and beta emitters aren't on your skin or ingested and present in your organs when they emit the alpha or beta radiation. There's not much you can do to prevent exposure to gamma radiation, but it's the least damaging.
Worry about cleanliness to avoid the worst types of radiation, and keep as healthy as possible to reduce effects (if any) from the unavoidable gamma radiation.