Moshe, Please point me to a halacha that says “mixed swimming” with your spouse is assur.
So chaverim and mosherose, does that mean we should all avoid anyplace with a warm climate? Israel has plenty of scantily clad people walking around. As does NY. So, lets all move to the Iceland (its beautiful there!) where it never really gets warm enough for people to walk around scantily clad.
Ask your rav and report back.