[Avodah] When does mixed swimming mean?

[Michael Makovi]

A friend of mine recently learned that there is no issur of mixed
swimming per se, and asked why then do we avoid it? I replied to her:

>>>>
Actually, it makes a LOT of sense. There's no halacha for davka
swimming; rather, you have halachot of tzeniut, which applies to
swimming no more than to shopping at the mall. Why should the addition
of water change anything?

Practically, however, I imagine it is difficult for women to wear
garments that don't reveal their shape. Even those swimsuits that
cover the whole body, are skintight. Moreover, even if she wore
flowing loose garments, we all know what happens when women wear wet
clothing.

I absolutely do not think that it is a fence on a fence on fence
[which is how she described it]; rather, it seems that practically, it
is impossible to keep tzenua while swimming.
>>>>

She responded to me, however, that there is no issur of wearing tight
clothing. This seems wrong to me, but to be honest, I cannot recall an
explicit source that tight clothes are assur. We have arguments about
showing arms and revealing a tefach and uncovered hair, but I cannot
recall (from Rabbi Henkin's handy summarization of every single Gemara
on tzeniut) anything about skintight. Is my memory faulty, or is my
friend correct?

Mikha'el Makovi