I did this trip from Cozumel in July, 2011, and I can tell you that the one-day trip from Coz WILL wipe you completely out. You start by catching a very early Ferry to Playa Del Carmen - 6:00 a.m.. Then, you get on a Van at PDC, which drives around picking up other passengers from surrounding Resorts and heads up to the Port north of Cancun - about an hour and a half sitting in the Van. When we got to the Port, we ended up sitting around for about another hour waiting for everyone on our boat to get there. Finally took off for the Whale Shark feeding grounds, and it was a long and kind of rough boat ride - over an hour, with a lot of bouncing around. Snorkeling with the Whale Sharks was a blast, but it was NOT for the casual or out-of-shape snorkeler - you had to constantly kick and kick hard to keep up with those magnificent beasts. I was physically spent. After a couple of hours of that, we went to Isla Mujeres where we could snorkel in a Lagoon off of a Beach Area, had a lunch with fresh made Ceviche, and generally kicked back. July is HOT, so I spent as much time in the Water as I could. Finally back to the Port, another long Van ride back to PDC, wait for the next Ferry back to Coz. I didn't get back to Coz until close to 7:00 that evening, and I was too tired to even go out to dinner, and was sore and wiped out the next day. I highly recommend the Whale Shark trip, but I would make it at least a two-day trip - either go up to Isla Mujeres the day before, or stay over on Isla Mujeres after the Whale Shark excursion. The other alternative is to use MayaAir to fly from Cozumel to Cancun & back - that really shortens the Ferry-Van ride, but is more expensive.