For a man, is it possible to tovel one’s self in the Mikvah while wearing a swimsuit? Is it considered a Chatzitza?
As long as it is loose fitting enough and not tied tightly at the waist, so that water can freely reach every part of the body, there is no problem of a chatzitza.
SHulchan Aruch Y:D 198:46.
See ibid 198:1, that for women’s tevila additional stringency applies and a woman may not generally be tovel with even loose clothes. In a unique case of need a halachic authority should be consulted.