Where did you see them exactly, in case I go back to Mexico one of these days?
Seeing whale sharks is a tourist industry in the Isla Mujeres/Holbox aggregation area during the summer. It's huge with dozens and dozens of boats going out every day taking hundreds snorkelers out to them. It's advertised extremely heavily in the Cancun/Isla Mujeres area with resellers hawking the trips on every corner and ads in every tourist magazine and all over the internet. Use google you'll get a ton of information.