Based at Puerto Juárez on the north edge of Cancún, where the Isla Mujeres ferries go from, running the whale shark trip between roughly May and September.
It is a nine-hour day: out past Isla Mujeres or Isla Contoy to wherever the sharks are feeding that morning, two snorkellers in the water at a time with a guide, and a long ride back. Groups are kept small. Starting from Cancún rather than the island saves the ferry but adds an early alarm.