If you’re new to Cozumel, it’ll be helpful to understand
the difference between “slow” and “fast” boats.
Indeed, it’s a big difference.
Undercurrent reader David Sifre (Bronxville, NY)
  recently went on Dive Paradise’s Paradise Diver, a craft
  that can carry 24-plus divers (although mercifully, they
  usually stop at 16). He reported that: “The ‘slow’ boats
  literally take an hour to get to the first dive destination.
  On our first day, the boat did not return from the second
  dive until 2:15, and the afternoon dive was scheduled
  for 3 p.m. The second day we made it clear we
  did not want to get back so late, but were told, ‘Relax,
  its your vacation’ as if taking a dive vacation meant we
  ought not be concerned about missing the third dive of the day! Finally, since seven of the 14 people on the
boat had a 3 p.m. dive, the crew did not go as far as
they had originally intended.”
While there is no agreed-upon distinction between
  slow and fast boats, the latter typically get you to dive
  sites in much less time, take only six divers (hence the
  term, “six packs”), tend to depart earlier and cater to an
  experienced crowd. They also cost more. Some operations
  run “fast” boats exclusively, and there is only
  one price. If an operation runs both types, it assesses a
  surcharge for the fast one; Dive Paradise, for example,
  charges $7 more for a morning two-tanker in its fast
  boat.But for serious divers, fast boats are well worth
  the added charge.
 - - Doc Vikingo