As I said, your lack of knowledge of the system outside your local lock is clouding your judgement.
You may love "your lock" and "your Avon" but you obviously have no experience of anywhere else. Kids swim in locks, lakes, quarries and rivers all over the country and rarely get into trouble, just as drunk boaters use locks throughout the system occasionally and don't get into trouble. As a mere member of the public you are in no place to tell people what to do, in fact doing so is likely to get you into trouble, especially from a group of stroppy teenagers. Even as an official CaRT volunteer I suspect your instructions would be to stand back and let them get on with it FOR YOUR OWN SAFETY.
If you want to "care about the community" then go and do the job properly, but volunteering with CaRT and doing their training.