The eco-parks such as XCaret and Xel-Ha require you to have biodegradable sunscreen as well as any of the snorkeling or diving tours. This is a federal regulation that is enforced. The biodegradable sunscreen at the parks is extremely expensive! I have provided links to a few biodegradable sunscreen brands. The site below is approved by the eco-parks and tours in the Cancun area. The bottle has "biodegradable" boldly printed for park staff and tour guides to see. They offer biodegradable packets for $0.99. They do not have travel size bottles, and the 8 oz. bottles are a bit spendy ($18). I think I will purchase a packet or 2 for each OOT bag. I will also purchase 1 or 2 bottle(s) that I will keep and let my guests know that I have it and they can borrow it if they take an excursion. This company also offers eco-friendly insect wipes and spray.
If you use USA Transfers from the airport, you can order bottles of biodegradable sunscreen from them. They will deliver it to you when they pick you up from the airport. It is $12 a bottle.
Here are some of the other good brands, although the packaging is not always as obvious and it may take some convincing staff that these truly are biodegradable.