As San Pedro is a resort and diving island, most of the best beach venues are resorts, especially if you have guests to cater for. As Judyann mentioned, it's also possible to have a wedding at a restaurant and of course boats have been mentioned, if that's your preference. Taking a boat out to a sandbar is getting popular, or, if it's just the two of you, a planner can arrange something lovely on a quiet beach somewhere. I believe there was quite a fun wedding at the Truck Stop, which is on the west side and it has a great dock with sunset views. I'd suggest you contact Melody at Dulce Belize Weddings. She's creative and fun and she will be able to give you the local assistance you need to get it done. You can find her on Facebook.