Certainly not an enviable situation. Best course would be to try the carrot approach to get the owners to act. Be their friend, the cops will be the stick approach.

Of course, if you're dealing with people who just see the place as a great spot to store their boat for the season, you're screwed.

Beyond that, best long-term plan would be to work with the officials (you want them to see you and the others who follow the rules as the "good guys" who appreciate the opportunity to use the spot).

Can the signage be improved?

Anyone without a sticker should be removed immediately, they are hurting everyone. Maybe give them a warning and 7 days to act, but nothing longer than that. If they aren't using the boat every week, they are just storing it there (especially if the rules are clearly posted and say no more than 2 days).

Anyone with a sticker should be able to be tracked (assuming these stickers have numbers that can be referenced back at HQ). Someone needs to contact them directly so they are aware of the problem they are causing. These boats could get the "nice" flyer, appealing to their sense of community. They also shouldn't get more than 7 days to act, for the same reason (it's not a storage yard).

All of this needs to happen quickly, before the police and other residents lose their patience and move to work against you by changing the program (I've seen it happen elsewhere).

Hope this helps.

Mike