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:
Although you may be thinking at this point that this seems doable, I must bring a 3rd point: