If you want to slather something on your legs, use baby oil. I dove with a DM in Utila who showed me her secret... A generous covering of baby oil ALL over any exposed skin. The oil trapped and killed any sand fleas/noseeums. It is yucky though and she would wash off as soon as we were on the boat. Also made for easy entry into a wetsuit. If you're staying at FI, then there is not too much to worry about. In the past, they would rake the sand daily to get rid of the fleas. Very few sand fleas on the beach but do take precaution.
But that doesn't help with keeping the dengue fever infected mosquitoes off of you... Dengue fever is very common there..