if you're going to the natural aquarium of Xel-Ha, bring a sunscreen and sunblock that is marked as biodegradable. you're supposed to use only eco-friendly produts, since you swim in a natural lagoon, not in a pool, and when regular sunscreens wash off your skin, they remain as a thin layer on the water gradually destroying the lagoon's ecosystem. but the prices inside Xel-Ha for these sunscreens are insane, think $25 for 4oz bottle.