I'm such a planner I would hate not to be fully prepared in advance. I read that alot of the beaches in the Caribbean have alot of sand fleas and you get bitten up pretty good. Is this true? I would bring repellent but I know it's bad for the ocean and the ocean life.
You could try Bull Frog Mosquito Coast. It is a water proof sun block with insect repellent. It is also DEET free. I've been using Bull Frog sun screen since before it was widely available in stores. I haven't had many problems with the sand fleas. The no-see-ums did a number on my legs once though.
I've posted this before somewhere, The Shore Excursion Desk sells cheap snorkels for $5. That is a good option for light packers that don't want to put the rental gear in their mouths.