Is it assur for an older lady fully dressed with SHVIM KLEIDER to go swimming with older non jewish and irreligious jewish men?
It is incorrect (and according to some poskim assur) for her to go to a mixed swimming pool even when she is fully dressed. The poskim say that it is a lack of tznius for her to look at the men, and to be in such a place, there is also an issue of maris ayin for her to be in such a place.
Rivivos Efrayim 2-197(25), Yabia Omer 1-pg. 20. Halichos Bas Yisroel 7-17.