might want to bring extra batteries also -if no electric than no charging up! Night Sky is awesome, I have seen a moonbow from San Pedro! you could hire a guide to take you up north away from the town if you want less man made light, or just out into the water if your equiptment could handle it. Actually a few nice spots at the end of docks don't have lights. Don't bring anything you can't replace IMO. Sand, water, heat, salt (even in the air) all take their toll on everything. Also if you have any cheap giveaway telescopes or books it is fun to leave things behind with new friends. We always brought extra hooks, line, hair things for girls, rubber bugs, t-shirts, hats, sunglasses, etc.
The trouble for me is finding something awesome in the San Pedro Sky at night and then thinking I will see it here - then all I see at night is clouds! They did clear for the lunar eclipse though! Belize it!