If the sun's so bad you can be burned even with sunblock, take some vitamin E capsules. Swallow them, don't just squeeze the liquid inside on your burns. That's ineffective. Take one before going ouside each day, and that will protect you (along with the sunblock).
If you do get burned, take one as soon as you can after that, and maybe another one an hour or so later, and it cuts sunburn pain wayyy down and promotes healing.
Don't forget your digital camera! We will expect pictures of you and David - and the island, of course.
Have fun and stay safe, but don't miss the chance to venture out to the part of the island where the real people live.