Resolution of this issue would be helpful – I, quite perhaps incorrectly, bifurcated the issue to include (a) swimming and (b) inappropriate viewing. Which leads, then, to a question of whether mixed swimming would be permitted where all parties are dressed in whatever we define as tznius –
So, let’s start with a proposition that one accepts the “tznius” bathing suits (yes, there are those who say that even the long shirt/sleeve/skirt suits can cling when wet, but for the purposes of this discussion, let’s operate with an assumption that the suits retain the proper level of tzinius at all times/conditions, and that the men (again, operating on a hypothetical) are similarly covered in an appropriate swim short and trunks of an appropriate length (knee length? board shorts? pick your poison) –
But – aside from the issue of appropriate clothing – is there an issur of being in the same pool with an unrelated person of the opposite sex?