My son who is practically potty trained always seem to pee in his trunks when we go swimming, to the seaside or use the paddling pool. He also sometimes encourages his younger brother to do it despite the fact he knows it's not allowed. I can often tell if he's doing it because he'll stop in the water, get a smile on his face and look down, before moving on. It's not a huge deal on the beach or in the paddling pool, but in public swimming baths, I don't really want anyone to see him doing it and to risk being thrown out or told off.
Any ideas of how reinforce that he shouldn't do this?