While I recognise you are narrowing in on the La Paz and Holbox Mexico options, if are serious about Whale Sharks you should also consider going a little further south, down to the
Tropical Island of Utila off the north eastern coast of Honduras. It's an inexpensive scuba diving destination and you are almost guaranteed to see a Whale Shark.
While Whale Sharks are regularly sighted close to the shores of Utila, these sightings are more frequent in the months March-April and August-September. There are no "special" or expensive whale shark tours operated in Utila since whale sharks are often sighted passing by the dive sites. If you are here for a week or more, there is every chance you will encounter a whale shark "by accident".
To improve your chances of encountering a whale shark, almost all the Dive Shops have a weekly "Whale Shark Day", when the priority is to look for Whale Sharks rather than get to (or return from) the dive sites on schedule. It also is possible to hire a local fisherman to take you out in his boat, specifically for the purpose of snorkelling with whale sharks. The local fisherman often fish "the boil" around the whale shark for bonito and other bait fish.
There are, not one, but 2 whale shark research centres based here in Utila, who offer opportunties for certified and experienced scuba divers to participate in their research efforts.
There is superb and inexpensive scuba diving in Utila, part of the Bay Islands group, Honduras. Getting to Utila is easy from Houston TX International Airport - flying non-stop either to San Pedro Sula or Roatan Honduras and then catching a connecting flight (from San Pedro Sula) or a ferry (from Roatan). Utila is way too small to have it's own international airport.
However, if you are looking for National or International Hotel or Restaurant chains, Utila is not the place for you since most accommodations and eateries are definitely at the budget end of the market, all owned and operated by local people. But then Utila is more of an authentic Caribbean island rather than a heavily built-out tourist destination.
For more information, check out;
Utila's Whale Shark pages
Utila's Scuba Diving Prices
Hotels and Places to Stay in Utila
Utila Dive Shops
Reviews from other visitors to Utila
or just follow the links from the
AboutUtila.com homepage.
I hope this helps, or at least gives you something to think about for your next scuba diving vacation.
Regards - Mark