The Sages forbade bathing the body in hot water on Shabbat. This is because some people were so eager to bathe in hot water that they would heat the water on Shabbat, thus transgressing both Hav’ara and Bishul. When they were admonished, they would claim that the water had been heated before Shabbat. Therefore, the Sages forbade bathing one’s body on Shabbat, even in water heated before Shabbat. However, one may wash less than half of the body in hot water. As long as one refrains from washing most of the body, there is no concern that one will succumb and heat the water on Shabbat for the lessened pleasure of bathing less than half of the body. Even when washing each limb individually, one still may not wash the majority of one’s body with hot water (Shabbat 40a; SA 326:1).
We have seen that one may use hot water already available in the boiler as long as this usage does not cause additional water to be cooked (above 10:24). We have also seen that many permit using water that was heated on Shabbat with solar power (10:25). According to this approach, one may use this water to wash the hands, face, and less than half of the body. However, one still may not use this water to wash most of the body on account of the rabbinic enactment.
The prohibition on washing the body is specifically limited to hot water. One may wash the whole body in cold or lukewarm water whose temperature is lower than body temperature. One may even turn on the hot water tap so that the water will not be cold, as long as this does not cause the cold water entering the boiler to be cooked (as explained above 10:24-25).
It is also important to note that the prohibition of washing the body in hot water is limited to normal circumstances. One who suffers greatly if he must go a whole day without bathing in hot water may wash his whole body in water that was heated on Friday or by a solar boiler (R. Akiva Eger; BHL 326:1).
One who bathes on Shabbat must take care not to wring out his hair with his hands while washing or drying off, in order to avoid the prohibition of Seĥita. He may, however, towel his hair dry. Since he is not interested in the water that is wrung from the hair and absorbed by the towel, there is no prohibition. A woman who always combs or brushes her hair after bathing must be careful not to wash her hair on Shabbat, in order to avoid the prohibition of Gozez (see section 3 above).
The Sages further cautioned that if one washes in a river, spring, or sea in an area where there is no eruv, he should shake the water off himself when he gets out and thus avoid carrying the water the distance of four amot. He does not have to be totally dry; it is sufficient to remove the excess water that drips from his body. In contrast, there is no rabbinic decree against walking in reshut ha-rabim when wet from rain. This is because rain does not get one as wet as bathing, and even if there is enough rainwater on one’s body that it could be considered carrying, the Sages were not stringent, since the rain falls on him against his will (Shabbat 141a; Beit Yosef and SA 326:7; Taz ad loc. 2).