This rash issue is raised at various hot springs every winter. Certain hot spring conditions and temperatures are conducive to the parasite that causes "swimmer's itch." When the air temperatures are cooler, visitors spend more time soaking than they do in the warmer months, and are more likely to be affected.
This parasite exists in some pools at Goldstrike Hot Springs in Nevada, but is not present across the river at Arizona Hot Springs (hotter pools with no surrounding soil).
The recommendation to alleviate the itching symptoms and rash is to towel off vigorously, immediately after soaking, and showering with soap as soon as possible. Since most people are far from home at natural hot springs, wiping down with rubbing alchohol or lemon juice seems to neutralize the parasitic critters on the spot. Rubbing alchohol also soothes the itching, if you've already developed the rash.
--NN