A family business run by Nano and Juan out of Sac Bajo on the lagoon side of Isla Mujeres, with more than twenty years on this water between them.
The season that matters is whale sharks, roughly June to September, when the largest fish in the sea gather north of the island to feed on plankton — you snorkel, you do not dive, and the rules on how close you may get are enforced. The rest of the year it is reef snorkelling and sport fishing, in groups or on a private charter.
They publish no street address, so this is pinned on Isla Mujeres town; confirm the dock when you book.