I did a W.S. snorkel trip with "MexicoWhalesharks.com" from Isla Mujures last Sunday morning. It doesn't matter whether you come from Mujeres or Cancun, because the boats pretty much zip between the two ports picking up passengers. We had people from both sides on our boat.
Worth the $130, I got to jump in twice, and both times I was swimming next to a pretty decent sized W.S. for about minute or so each time. Got GoPro footage too. Luckily I had one that wasn't super fast, so I could "almost" keep up. Our boat had about 9 passengers, 1 captain, and 1 guide that swam with you and the sharks). All our tourists got to go twice. Except for me it was all husband/wife couples and the guide.
Basically there looked to be about 25 to 30+ boats out. It took about an hour out in the Holbox W.S. zone before all the boats converged on the area where they spotted them. At first it was complete "semi-organized CHAOS" with boats almost on top of each other, but the captains were pretty experienced, no bumps that I witnessed. After awhile, groups of 4 or 5 boats would gather and try to form a sort of "line" or "order" for each of the dozen or so sharks out there. "Park" in front of a shark so that the sets of divers/guides could drop in front and start swimming. The trick I guess was to watch as the shark was coming towards you, then try to turn around and start swimming at speed, before you got to the gills, along side of the shark, then try frantically to keep up, keeping 6+ft. away. After the first time, it was pretty easy, even though it sounds complicated.
I'm glad I went, it was totally worth the money, and the only downside was obviously SO MANY boats and people. So we only got 2 turns each, and each turn was only from 30secs. to a minute "next to" each shark. But still worth it! Yes I'd do it again, but hopefully be able to dive, not just Scuba, like in Belize or similar place. Yes I still want to visit the Atlanta Aquarium and do that dive too, despite the huge $325+ price.