Finally today, as I approached the lake, I saw him up ahead of me and figured I'd get to see the plane in action. I was right. It's a seaplane, and it skimmed along the water, then went almost straight up. Then it dove straight down into the water. Pretty sure the guy's doing it on purpose, but why?
I could walk up and ask, but it's one of those things that's weird enough that I'm afraid to.