Question--Creeper at the beach

So, living by the beach, path, and busy street, I see a lot. One of the most frustrating things is the creeper. He likes to use his binoculars to spy on couples who are minding their own business on the beach. I spotted him by sheer accident one day, standing behind a bush among the dune grasses. At first glance, he appeared to be admiring the scenery through his binoculars. But, he kept staring in the same direction for long stretches of time, and there wasn't any scenery of particular interest in that direction. He would occasionally turn 180˚ and look toward SF, but he would immediately swing back around again. There was just something disturbing about his posture and his behavior, ducking and moving around for a better vantage point, and I finally located his targets on the beach--a young, amorous couple on a blanket. Now, granted, they're out in the wide open and so is he, for the most part, and there were many other people around, so I kind of just brushed it off. But, when they left, so did he...immediately. I've seen him a few times since then, always acting the same way, and when I see him, I know I'll see a couple on a blanket within his sights...and I always do. Today, I saw him again, and this time I let him know I saw him. He wasn't sure if I knew what he was up to, so he tried to be very nonchalant, but he definitely kept his eye on me keeping my eye on him. Frustrated, he moved from his vantage point on the walkway hidden behind a clump of grass, into the dunes closer to the couple because after he saw me, he felt uncomfortable using his binoculars. He moved behind a tall stand of dune grass and continued to peer around it at them, pretending occasionally to stretch and look in other directions, and of course, at me several times. The other man stood a couple of times, and creeper would suddenly crouch down in the grass to avoid detection. There is no doubt in my mind this guy is a peeping tom. Ultimately, the couple left, and creeper pretended to exercise and stretch long after they left, which he usually doesn't do. He finally left without looking in my direction, which I was hoping he would because I was going to let him know I know what he's up to. So, should I call the park police next time I see this guy? I'm not sure there's anything illegal, per se, about what he's doing. Perhaps I scared him off today, but this kind of guy won't stop this behavior...he'll just change his tactics or location. Unfortunately, the park police can take an excruciatingly long time to arrive, too. But, it's on park land, so I can't call APD...or can I? Could they detain him until the park police arrives?