618. Bathing on Shabbos

86:1 One may not wash his or her entire body, or even most of the body, in hot water on Shabbos. This is true even if the water was heated before Shabbos.  Even to wash one limb at a time is prohibited if one is going to wash  most of his body. One may not even go to a bathhouse for a sweat (like a sauna). One may, however, wash his hands, face and feet using hot water that was heated before Shabbos. (Some authorities permit a person who is experiencing great discomfort to bathe in hot water that was heated before Shabbos – Bi’ur Halacha 326:1 s.v. bamayim shehuchmu mei’eirev shabbos.)
86:2 Regarding hot water that comes from natural hot springs, if they are still in the ground and the spring is not covered by a roof, one may wash even his whole body in them. If they are in a vessel, or if the spring is covered by a roof, it is prohibited. (The ceiling retains heat – MB 326:10 – making it) like a bathhouse, which is prohibited. Some authorities permit one to bathe in hot springs in the ground even if they are covered by a roof. (According to Mishnah Brurah 326:11, one may rely on the lenient position in this matter.)