It's because water blocks radioactivity. It's like the opposite of the Fallout games and their radioactive water: you would have to swim right down to the radioactive material and wrap yourself around it at which point you'd basically melt.
Water blocks alpha, beta and gamma rays, but the water itself can carry radioactive elements, which I'd guess was the source of this (relatively minor) contamination.
Reactor h2o itself does not carry radiation, but any extra molecules in tend to do it, thats the reason why the water is as clean you can get, over-distilled. This by itself means that it is not potable (btw for disposal to environment it gets re-salinated), so they told the story of professor drinking it must be an urban myth. It is bad even for skin expose (swimming in it), but hopefully that worker got just a few seconds expose and is well.
Source: training trip in a nuclear center.
I was reading accounts from the survivors of the atomic bombs dropped on Japan. One of the survivors was blown out of the house and was stuck neck deep into water. Couldn't get free, so had to wait for rescue. They didn't get much of any radiation sickness afterwards.
