The difference in density between tap water and pure water is extremely small. Like, on the order of a few 1,000ths of a percent. If they had trouble staying afloat, it was likely due to the fact that they were wearing all of their normal work gear, plus the relevant PPE for being next to the spent fuel pool. Pants, shirt, maybe safety shoes/boots, gloves at least on their person, electronic dosimeter, etc. The shoes/boots are the biggest issue, they'll almost completely rob you of any ability to use your feet/legs to help keep you afloat unless they're designed to be flexible in the ways useful for swimming (which work shoes/boots obviously are not).