One of the most memorable thrills of my lifetime — there have been too many to easily remember — was my first swim with Atlantic whale sharks off México, in the Straits of Cuba, in 2006. My guide on the boat deck motioned to me, treading water and scanning the horizon for any dorsal fins, to “dive, dive!” and I did, just in time to see a shark some 30 feet long, with an open mouth nearly 5 foot…