Dont Swim

in the Canals. they might not like you giving them a lecture about the dangers of swimming in them but its just might help
Best and only way to stop this going is, if the adults who live nearby explain to them about the dangers, just think how bad you would feel if you woke up next morning to hear the news that one of them had drowned,
And Im sure some of their parents would appreciate it if you let them know what the teenage children are up to.