When I say that swimming in a marina is "stupid", I'm not talking about divers who clean boats in a slip and I'm not talking about people swiming around private docks, I'm talking about swimming back and forth across the fairway with boats coming and going. The risk is being hit by a moving boat or causing a boat operator to wreck while trying to avoid the swimmer.
I understand how a ten year old child might not be able to think this out, but his parents should be able to. So should older children.
We have this situation in my marina from time to time when dry stack boaters take a slip for the weekend.