One of our longtime travel writers also dived with the
Yucatan whale sharks in August, and here is her report.
I went with Andy Murch, owner of Big Fish
  Expeditions ( www.bigfishexpeditions.com ), with
  whom I traveled last year to snorkel with sailfish. I
  stayed at the Na Balam Hotel on Isla Mujeres ( www.nabalam.com ). Each morning, my group of eight had
  the included continental breakfast around 7:15 a.m., then
  waded out to the boat. About an hour later, we were
  usually one of the first boats to arrive at our destinations.
  We spent four days snorkeling with whale sharks,
  manta rays and a few smaller mobula rays. The first and
  fourth days were mellow, but days two and three were
  kick-ass, with four or five whale sharks often in view at
  the same time, and sharks and rays swimming side by
  side. I had to continually look in all directions; the whale
  sharks seemed to enjoy running us over if we weren't
  paying attention!
My group was met at the Cancun airport by Na
  Balam's Andy and Laura, then taxied 30 minutes to the
  ferry depot for a 20-minute ferry ride to Isla Mujeres.
  Two-thirds of the hotel is on the beach side of the road;
  I was on the other side of the road, with the small pool.
  My room was large, with plenty of storage. It was airconditioned
  but the maids turned it off during the day, and it took until late evening to cool down sufficiently.
  Na Balam is a couple blocks from the main town, where
  I enjoyed different restaurants each evening, usually
  with outdoor dining. One complaint: the extremely
  loud music that carried on until 2 a.m. on Saturday and
  Sunday nights from the Fenix bar (not part of the hotel)
  across the street from my room. Get a low-numbered
  room far, far away from the loud music.
By 9 a.m. each day, there were 30 or 40 boats spread
  over a wide area, so there wasn't really any crowding;
  just wait and the sharks come to you. I had read that
  only two snorkelers from each boat were allowed in the
  water at the same time, but we all jumped in together,
  remaining for up to two hours, then we climbed into the
  boat for food, water and rest. Solo Buceo, the dive operator
  ( www.solobuceo.com ), provided plenty of water,
  sandwiches, soda, fruit drinks, cookies and even beer.
  Then back into the water for a couple more hours. By
  2 p.m., the whale shark activity started dying down as
  they went deeper, and we headed back to Isla Mujeres,
  exhausted and happy.
Big Fish Expedition's trips next year are $1,960, double
  occupancy.
-- J.D.