Other than the possibilities of immodest dress and behavior, and the more general issues of the mixing of the genders (see Masecheth Succah 51b and, e.g., Igroth Moshe, Orach Chayim I:39), there is no specific prohibition inherent to the physical act of swimming. Obviously, clothing that might be modest when dry might not be so when wet. And generally, swimming is an act of leisure more likely to lead to inappropriate interactions than would the mixed funerals mentioned in the aforementioned gemara, which means it is more of a problem certainly than mixed seating at a synagogue. But as far as specific swimming related guidelines, that would be up to the contemporary Rabbi ruling on the diverse circumstances involved in a specific situation.
(Anecdotally, Morasha Kollel is a religious, men's learning camp, under the auspices of Rav Mordechai Willig, in the Poconos that has swimming for the men at one end of the large lake simultaneously with swimming for women at the main camp at the other end. Swimming is limited in both areas to a small swimming area and both parties arebarelyare barely visible to each other due to the great distance.)
