Fishing for mermaids
Was going to put this in Silkstockingslover's thread but I guess this idea doesn't have to be a summer idea.
In a town, a prank to pull on people, especially outsider or ones who are hard up for sex, is to tell them to go "fish for mermaids" in a secluded lake. The town has a lot of legends about mermaids and there's a few drunken sightings each year. The fishing is actually pretty simple. Just wade out until you are neck deep or can sit down in the water till only your heads above water. Whatever you do, don't turn around or duck under the water, it'll scare them away
...oh, and leave your trunks on the shore. Mermaids don't like fiddling with them. Just wait a while and one will come to suck you off.
Of course what usually happens is someone just runs and snatches up your trunks. Even if it doesn't, animals, the wind, or something else comes and takes your trunks, leaving you naked.
But what if someone went out there and suddenly found their knob getting swallowed? Could it actually be a mermaid, someone taking pity on the poor sod who falls for the prank or something else (no bestiality though)? And what if some curious woman went out there or lost her bikini bottoms while swimming?