How important is it to bathe in hot water on Erev Shabbos? I know someone who goes to a cold outdoor spring as a Mikveh on Erev Shabbos and does not bathe in hot water saying that it is not necessary and is a mere stringency and even showed me from the Kitzur Shulchan Aruch that one is not to do so.
It is a Mitzvah upon both men and women to bathe their entire body in hot water on Erev Shabbos and Erev Yom Tov in honor of Shabbos and Yom Tov, and at the very least to bathe their hands face and feet. This is a clear ruling in the Talmud and Poskim. One does not fulfill his obligation with bathing or showering in cold water. The above statement from the Kitzur Shulchan Aruch is inaccurate and is being taken out of context. Nonetheless, what is true is that this bathing [even of the face feet and hands] is not an actual obligation, but rather one who fulfills it is rewarded, while one who does not is not punished.
Sources: See Admur 260:1; Michaber 260:1; Gemara Shabbos 25b; M”B 260:4; Biur Halacha “Bechamin” Ruling of Yaavetz-Not to enter entire body into hot water: The Yaavetz in his Siddur rules that one is to bathe his hands, feet and face in hot water. One is not however to enter into a bathhouse as doing so can cause difficulty in marital relations which is supposed to be performed on Friday night, as is written in the laws of Derech Eretz. Practically, Admur does not mention this ruling of the Yaavetz, despite him mentioning other matters [i.e. eating garlic] which are to be done to enhance Tashmish, and we thus see he does not suspect for this at all. Therefore, it remains a Mitzvah upon all to bathe their entire body in hot water on Erev Shabbos as rules Admur. Furthermore, even according to the Yaavetz if one’s wife is not pure, he too would agree that one is to bathe his entire body in hot water. Furthermore, even when she is pure it is likely that the Yaavetz was referring to the bathhouses of the past which consisted of an entire lengthy bathing ritual of sauna, steam room and hot tub. Thus, mere showering in hot water was not being referred to, and hence remains a Mitzvah upon all to perform. To note however from Kitzur SH”A 72 which states that the Yaavetz rules one is not to bathe his entire body in hot water without mentioning the idea of a bathhouse, as is the original words of the Yaavetz. However, see also the following Poskim that the issue of bathing in hot water on the day of marital relations only applies to excessive bathing: Makor Chaim of Chavos Yair 240; Aruch Hashulchan E.H. 25:3; See Upikadeta Navecha p. 51. And many Poskim also rule that the matter is not forbidden from the letter of the law, and is just a health adherence: M”A 240:29; Elya Raba 240:23; M”B 240:54