The operator to go to for Isla Contoy, the uninhabited national park thirty kilometres north of Isla Mujeres where more than a hundred and fifty bird species nest and visitor numbers are capped by permit each day. You cannot simply turn up; the trip has to be run by a licensed boat, and this is one.
They also run the whale-shark snorkel in season, all-inclusive island tours and a sunset cruise, both shared and private. No shopfront address is published, so this sits on the town — the boats leave from the Isla Mujeres side.