Why shield radiation with both water and lead? An equal weight of lead stops more x-rays than an equal weight of any lower element on the periodic table -- lead's about the highest "Z", or highest number on the periodic table that you can get without getting too expensive. But not all radiation is x-rays: some are charged particles, for which you want a lot of lower-Z atoms -- the more atoms the better. For maximum number of atoms per gram and maximum atoms per volume, it's hard to beat cheap old water. (It can be beat, of course, but then the expense goes up...)

It gets a little more complicated, because when a shield (e.g. water) stops a charged particle, energy is released as secondary x-rays. So the lead-lined wetsuits in this story are useful in stopping the "secondary radiation" created in the water tanks itself.