meh, it's a plot hole. If said spring existed in jusenkyo, the very moment someone falls any spring, the guide would tell them, "Come here, customer, jump into this spring, See, no big deal".
Or in the case of the girls, is even easier, since we know where is the "Drowned girl" spring. After all, ranma fell into it.
IMHO, there's only two possibilities. Either it's a cure and the jusenkyo guide, Cologne, the and most of the cast misplace their minds do not realize they could do this (Unlikely, the Jusenkyo guide should be aware of the possibility at the very least)
there is an adverse affect in doing so (Most likely scenario), (mutations, the curse locks, the curse mixes... )