תלמוד על שבת 217:10

⁷¹Babli 109a. One may bathe in the ocean and in the waters of Tiberias even though one intends to do this for medical purposes, but not in water used for soaking flax or in the Dead Sea⁷²Since everybody bathes both in the ocean or in the Sea of Galilee or the hot springs of Tiberias also on weekdays, nobody will notice that one bathes for medical reasons. But in a natural pond used to treat flax or in the Dead Sea one bathes only for medical reasons; this is rabbinically forbidden.. When? If he intended this for medical purposes; therefore to lift himself from impurity to purity it is permitted⁷³Using the pond or the Dead Sea as a miqweh cannot be forbidden since it is biblically justified.. Rebbi Samuel, Rebbi Berekhiah’s brother said, on condition that he not tarry⁷⁴Since immersion in a miqweh is done very quickly, staying in the water for more than a minimal time would be proof that the bathing was for medical purposes..