Just did the cruise/dive.  It is a hassle.  The dive shops hired by the ships are

cattle boats.  Dives are fast and shallow.  I booked my own in Coz, but
booked thru the ship in GC.  Remember when doing your dive that there is
 a time difference between ship & local time.

We just did that route in June with Carnival.
We used Neptune Divers in GC and Dive Cozumel - Yellow Rose in Cozumel.
They can be found at:

http://www.neptunesdivers.com/

http://www.divecozumel.net/scubarecreati.html

They both respond quickly by email.  Same day most of the time.

You can see pictures from the dives at Tinyurl.com/l52n

George

the bull asked:

In addition to George's excellent recommendations, I have a couple more to add.

In GC, Wall-to-Wall Divers and Off The Wall Divers also seem to do a good job
for cruisers.  However, you have to really do your homework about the logistics
since the ships tender in here and time can be a definite factor.  Sometimes
the best option is to go with the ship's excursion (and it pains me to say
that).

In Coz, much of it depends on the style of diving you want to do.  For a lot of
options, you can take a look at Dive Paradise.  As the largest outfit on the
island, they have boats going out at various times that can accommodate your
wishes.  If you want a smaller shop that has built its business catering to the
cruise trade, but doesn't run a cattle operation, try Eagle Ray Divers.
They're a friendly shop who give a lot of personal attention.  Finally, Papa
Hogs is quite cruiser friendly and gives you one more option in finding the
right fit.

Amber