I spent a month in southern Mexico, more precisely in Yucatan and Quintana Roo, beach bumming, snorkelling and eating guacamole-based foods.
As I was working meanwhile (margarita in hand), I travelled super slowly and also decided to stick to the rather touristy areas, as I was relying on a working internet connection.
See the art at Isla Holbox
Take your instagram pictures in Tulum
Mexicans seem to be obsessed with Bacalar, as everybody I met told me how much they loved it.
There are beaches where you'll find tons of starfish, you will definitely see nurse sharks and turtles when diving and the entire island is focussed on exploring the ocean surrounding it.