One unique underwater event that doesn’t get much
publicity is the winter schooling of sailfish off Cancun.
John Kontnik (Lakewood, CO) and his wife dived there in
February and reports:
It was the real deal but one of the roughest trips we have
  done. There are lots of sailfish but getting into the water with
  them is the trick. We spent seven days on a 33-foot boat in
  big seas with 10- to 12-foot waves, looking for the schooling
  sailfish. You leave the dock at 6 a.m. and return about 4 p.m.;
  a long day on a small boat. No bathroom -- all your business
  is done off the back of the boat. Poor food -- I hope I never
  see a ham and cheese sandwich on white bread again, as we
  had them for breakfast and lunch. There is a great deal of
  competition between the two snorkel boats and the 50-plus
  fishing boats in the same waters. We are all looking for the
  same thing -- flocks of birds near the surface.  
Once you spot the birds, everyone rushes to the site. If
  you get there late, you’re out of luck. Because the snorkelers
  are getting in the water, you can only hope the bait ball is
  slow-moving. If it isn’t, you can’t get in the water. You can
  go days without getting wet. However, if you get lucky with a
  calm day and not much competition, it’s spectacular. We had
  one such day. We jumped in the water and when the bubbles
  settled, there had to be 20 sailfish herding a small bait ball.
  The sardines approach you looking for shelter and the sailfish
  follow. I found myself 18 inches from the sailfish but they
  didn’t seem to be upset by my presence at all. This went on
for 30-plus minutes, then for several more times that day. 
Other encounters on other days would last from less than
  a minute to several minutes. Most action was on or near the
  surface, because the sailfish herded the bait fish there, so
  tanks weren’t really necessary; we snorkeled and free dived.
  There seemed to be a protocol between the fishing boats and
  snorkel boats: 10 to 15 minutes, then get out of the way.  
We were fortunate to have had that one special day. Other
  people we met had made the commitment of four weeks-plus
  on the boat because of the random nature of the encounters.
  Both of us came home with bangs and bruises from the
  rough seas but you play, you pay. Go with full knowledge that
  you may strike out but if you hit a home run, it’s out of the
  ballpark.  
Amos Nachoum arranges trips (www.biganimals.com) but
  we thought the whopping $7,900 per person for nine days too
  expensive, so we did it for half as much. We stayed at Hotel
  Playa la Media Luna (a little outdated but convenient to the
  dock and downtown), which cost around $1,100, double
  occupancy, for eight nights. Four of us chartered the boat for
  $10,950, or $3,550 each. We used Keen M Sports Fishing for
  the boat (www.islamujeressportfishing.com); only a couple of
  boats do the sailfish snorkel thing. We rented a golf cart for
  $65 a day to carry gear. Yes, it was pricey, but I was able to
  put together a 15-minute video with some great stills my wife
  shot. When we watch it, we still get chills.  
P.S.: Isla Mujeres itself is fun and funky. We were lucky
  to be there during their Carnival, and lots was going on in
  town. Not that we stayed out late: I was so tired that the bed
  looked good at 9:30.