Oh joy..
I called an insurance agent from Island Insurance, and she stated that for events such as weddings, where they expect 25 people to show up at a beach and attend the ceremony, it would cost a couple hundred dollars.
In such an event like this where I don't really have control over the situation, especially if we're holding up signs and waving at traffic, she said it will easily go up into the thousands of dollars. I expected to pony up my own money, but I won't be able to afford that. Island Insurance said no one on island will be able to underwrite it, and it will have to be someone on the mainland. Also, they will want to know exactly what signs will be waved and exactly what we want made known, among other things. I'm waiting for another email from the insurance agent, but it's not looking good special-use-permit-wise...