My wife and I did a Whale shark trip in June. We were staying in Akumal and signed up late with the dive shop we have dove with before (Blue Caribe out of GORM resort). We had to drive in our rental behind there van, but they gave us a discount. Roads are very rough to Chiquila and our tailpipe came disconnected from the engine.
Other than that, it was a fantastic trip and worth it. Only complaint on the trip is many people did not respect the whale sharks and the admonition not to touch. There were many boats out there, but the rule is one boat per whale shark. We were guaranteed at least two shots with the Whale Sharks, but got four. Difficult getting photos due to the plankton in the water, but what an experience! These are huge, majestic creatures. Took my breath away.
You cannot dive with them, only snorkel. Saw dophins on the way out. Small jelly's during the snorkeling. A woman we know saw a Manta playing with a whale shark last summer. Holbox is a delightful island. We wish we could have stayed there a few nights.