To purchase this Sefer, click here
One does not need to cover his body in a bathhouse. The same applies to a river. (Nevertheless by a river one should get undressed and dressed as close as possible to the river in order not to be uncovered unnecessarily). [From here it is learned that one should not leisure around even in a bathhouse when he is not dressed. Based on this some are particular to remove their under garments close to the Mikvah.]
Covering the Erva: Upon descending to the river one does not need to cover his private part, and one who does so appears to be embarrassed [of his circumcision] and its considered as if he denies the covenant of circumcision made with Abraham. However when ascending [from the river], being that one is now facing towards people, one should bend [in a way that will prevent his private part from being seen], or should cover his private part with his hand in a way that he will not touch it. This likewise applies when one is bathing in a bathhouse together with other people. If it is possible to cover one’s Erva from the eyes of others using a cloth then this is the best option.
It is permitted to be unclothed in a bathhouse or river. Nevertheless by a river one should get undressed as close as possible to the river. Upon ascending from the river/bath one should his cover his erva with his hand or a cloth.
May one remain not fully clothed by a beach?
One may remain in the water undressed although he is not to walk around the beach with his body uncovered.
Is one to cover his Erva when bathing in a Mikveh?
One is to cover himself when returning from the Mikveh back into the dressing room. However when entering into the Mikvah from the changing room it is unclear from Admur as to whether one is to cover himself in this instance. Practically the custom is to be lenient not to cover oneself at all in a Mikveh. Although meticulous individuals are careful to cover themselves.Alternatively they remove their undergarments only near the Mikveh.
 Basra 2/2; M”A 2/2
 The Reason: An exception to the above rule of modesty is by a bathhouse or river, as in these places it is common to walk around naked, and it’s not possible to be clothed. Thus being unclothed in such a place does not break the code of modesty. [ibid]
 Parentheses in original
 Shulchan Hatahor Mamar Tznius 4
 Az Nidbaru 5/40 and 50; Piskeiy Teshuvos 2/2
 As Admur ibid rules one is to cover himself when facing the people, and he then writes the same applies by a Mikvah.
 By a river Admur differentiates between one who is entering the river and one who is returning from the river. He then concludes that so is the law of one who is in a bathhouse with other people. It is unclear if his intent is to say that one is to always cover himself in a bathhouse, as he is always facing people, or only after returning from the water. Vetzaruch Iyun.
 See Piskeiy Teshuvos 2 footnote 11
 Likkutei Mahrich Seder Rechitza Erev Shabbos