What are the issues with mixed swimming, from a tzniut point of view? Does it make a difference if the swimmers are or aren't Jewish? For example, are there issues with me swimming with my wife and a non-Jewish male friend, provided that both me and my wife are modestly dressed? Does it make a difference whether it's a small body of water (swimming pool) or a large one (lake or ocean)?
TK, although I cannot definitively answer your questions, I can touch on some of the issues that I have discussed with an authority:
"Mixed swimming" is a subset of forbidden relations (abizrayhu d'arayos)- loosely translated as promiscuity. However this would apply only when in the pool in proximity, but not when a male is on one side of a large pool and the female is on the other. I don't know of any distinction between Jewish and not.
Another issue that would apply at all times is stray thoughts about women. Again, I don't know if it makes a difference if the "thinker" is Jewish. I was once involved with a group who were going for advanced swimming lessons, using a separate part of the pool and where thoughts were less of a problem because of the age of the other females in the pool. I was told that I did not have to protest.
Although you may be thinking at this point that this seems doable, I must bring a 3rd point:
- Based on the Shulchan Aruch EhE 21, those who are careful about mitzvos avoid mixed social events, especially where the sole purpose of the event is to have a good time together.