Here is a controversial thought.
Put out a video camera with wide angle lens, every time the club meets. You don't need permission to video in public places in the UK. I would not ask the council permission as they will probably say no, even though permission is not required, as lots of people video in parks.
Then in the unlikely event that there is a problem you at least have some evidence that it was not a club member. Especially if every member makes a point of walking in front of the camera with their boat as they go out. It would not be 100% proof that it was not one of your members, but would go a long way, especially if you do capture images the perpetrators either doing the act, or just walking by with the problem boat.
Some of the new action and 360 cams have very high resolution.
The camera would not be permanent, just put it out on a tripod, and as long as someone has an eye on it, it should not get stolen. You will only need to keep footage if there is an accident. An additional benefit is that you can capture members sailing activities and share them. If you do get some Karen wanting to know what you are doing, just tell them you are filming club sailing activities, but because the camera will be passive, probably no one will even notice.