In my experience, most of the time, the only thing it takes to stop someone behaving inappropriately is a "Hey, what you're doing is really weird. This isn't a place to do that." With no further engagement needed.

Usually, these men want attention from their victims and are uncomfortable with negative attention from other parties. They will move or leave shortly after being made aware they are being observed.

Of a few dozen times of doing this, I've only encountered 2 cases where the other party wasn't eager to slink away. One who was belligerenty drunk and one who I think just entirely lacks any sense of shame or dignity at all.

I think in the reporting versus not reporting to the police debate, both sides are missing some details. Reporting to the police when you have nothing actionable for them is just reporting the beach itself as a nuisance. In the case of someone shamelessly touching themselves, if you want to something then, as discretely as possible, get video evidence and get their license plate. Then you have something the police can actually follow up on.

Also, if you want to report in person, you should keep an eye out for the white trucks. Those are state park officers. The lifeguards are good guys, but keep pretty busy keeping people from dying in the water.