We've had good luck using Craig's List. Post in the activities as well as the boats for sale section. Sounds like your location might benefit from fliers around the launch area as well.
Find a local boating shop to sponsor some kind of BBQ. Cheap, like hot dogs. Set up the Q, and make a family day out of it. Getting the families involved is key. Have a central location where people can meet and gather information. Make sure that you acknowledge everyone. I guarantee that a lot of those boats are sitting idle because people feel like they're on the "outside" of some sort of sailing fraternity. Make 'em feel welcome!
You might even do well to print a couple of fliers and canvass the houses where you know the boats are.
Also see if a local sailing /boating shot, sailing club/yacht club, will line up a "guest expert" that knows the area, and the boats. We had the Coast Guard come and talk at a sail day for example. We hosted a "Safety Seminar" one time where we taught everyone how to right their cats. It was wildly popular, and we had about 50 people show up on short notice.
Do it on the cheap and have fun. It's like organizing a race. Just tell everyone what's going to happen, put a little organization into it, and people will show up and have a good time. The second time you do it, it will be even better!