1. It's explicit in the title: you get to swim with whale sharks (Rhincodon typus). It's not shark watching (although the behemoths are often easily visible from the surface): you get to get up close in the water with the largest fish in the world.
2. This is an opportunity to have a different interaction with a fish than just eating it. You can look into its eye and appreciate its harmlessness, size, and speed. You can see the gills in action.
3. Whale sharks are absolutely harmless. They filter-feed on plankton (you'll even get some in your hair), which is what brings them to the surface. They also bear live young, which is part of their vulnerability.
4. Do it while you can. Whale sharks are listed on Appendix II of CITES. They are internationally traded for their shark fins and also wanted for their meat. Read more about their status and threats official CITES documents.
5. Support the tourism industry. The local economy of Isla Holbox, Mexico, where I visited, was built upon whale shark tourism (and it's fairly obvious they are more valuable as spectacles than as soup). I was also impressed by their rules: only snorkeling is allowed, swimmers have to wear a wetsuit or a lifejacket so that they stay buoyant and surface-constrained, no touching, and only two people in the water (plus a guide) with the shark at a time.
6. Afterward, you'll want to support conservation groups doing great work to save sharks, like WildAid, BLOOM, and Vancouver's own Shark Truth (whose founder, Claudia Li, went with me to Mexico last month and is the co-star in the photos here; see an article that features her work today in the Vancouver Sun about the certification of a BC shark fishery -- another questionable move by the MSC).
7. There are other great things to do. Not far from Isla Holbox there are cenotes, Mayan ruins, and diving.