Men should feel,uncomfortable swimming when women are watching. Bathing suits are water wear underwear and I hope men would feel uncomfortable in the presence of women if all thye had on was underwear.
Tniyus means acting in sucha way that you are aware of Hashems presence. Hatznaya leches IM Hashem, walk modestly with Hashem… modest is the characteristic of Hashem, one when we emulate it invites HIS presence.Just as one cannot say a bracha when wearing a bathing suit unless he covers himself up because of “erva” one should not be in the presence of women in such a state. Women should not make men feel uncomfortable by being around the pool……. it is unfair and wrong.