I knew this lifeguard who said she'd have to breakup couples having sex in the lake at least once a day on the weekends. She said the guy would always get super pissed and glare at her when coming in from the water. I had to ask what you yell at a couple that's having sex in the water at a crowded beach, and she said she'd blow her whistle and yell, "Cool it!" That seems like a pretty good approach since the other beachgoers would likely think she'd been yelling a group of teenagers splashing each other. After all, it's not like there are any chemicals in that lake to kill stuff, and you don't want people freaking out.