The twin issues are: (1) mixed swimming, and (2) seeing ladies “uncovered.” The latter, in fact, is problematic when visiting various amusement parks, since women (particularly in parks with water attractions) tend to walk about dressed for the occasion.
Without imposing my view on anyone else, I once asked my rov whether, if I went to the beach, the issurim of mixed swimming fell away if was 50, or 100, or 150 yards from the nearest woman. His response was clear, and to the point: “If you’re close enough to see anything interesting, you’re too close.”