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.
