So there are a few different things that we use water for.

Water is a great coolant.

It also makes a good moderator in many designs.

It is an effective shield for radiation sources. About 7 feet of water will reduce the lethal radiation levels in nuclear fuel down to levels we can work under. The spent fuel is typically under 23 feet of water to act as a buffer in case a fuel rod leaks or splits open to act as a dissolving agent for radioisotopes that leak out.

The last part though, is the water can have radioactive material dissolved in it. So yeah it would shield you from the radiation from fuel rods 23 feet deep. But if there are dissolved fission products in the fuel, when you jump in the water those products are now coated on your skin, causing direct radiation impact. If you ingest it or your body absorbs it, you can have internal effects.

So while it’s not going to be lethal like hugging a fuel rod, it’s still harmful and we need to decontaminate you to not only protect you, but keep it from getting out of the plant.