I was told by a chemist friend that what was most dangerous was sources of ionizing radiation, which, if taken in, for example, through ingestion or inhalation, would often be sequestered in the body close to sensitive areas. That suggests that the situation for a diver might be much worse — due to long term exposure to radiation — if radioactive material leached into the water and the water was accidentally ingested
This concern seems to fail to understand exposure.
If sufficient material had leached into the water such that accidental incidental ingestion were a long term concern, that means that the water would be radioactive enough that the immediate exposure due to immersion would be the hazard of concern.
What happens if you drink it only matters if you aren't otherwise exposing yourself to lots of it.