Q. Are a couple permitted to swim together if the wife is a Niddah?

A. Poskim prohibit even preparing a bath for the spouse when the wife is a nidda (See Shulchan Aruch Y,D, 195, Shach (16) and Taz (8).
Therefore, Horav Shlomo Miller’s Shlit’a opinion is that it is prohibited.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a