Check out this article on Essential Travel Items for Mexico for a full list of things you will want to have with you.
Essential Travel Items
Biodegradable sunscreen is a must, as is a dark pair of sunglasses. You can rent snorkel gear and water toys (kayaks, paddle boards, etc) everywhere. You will want to spend a lot of time outside in Akumals natural beauty, so make sure you have a bottle of water at all times and a good hat.