More info ... Only the three DMs were in the water at the time due to some confusion about the dive site. The captain's GPS didn't have power (no batteries or dead batteries) so the DMs were looking for the dive site.
Seasoned Captains and Divemaster (3 of them couldn't find it???) do not need a GPS to find the standard dive sites here in Cozumel - and very, very few use GPS unless diving remote sites or new sites. This goes back to an entirely different thread from a month or so back about the level of experience a crew on a boat has, specifically referring to a couple of the large resort operations that hire temporary DM's and Instructors as part of their DM and Instructor training programs - this is a good example of why a seasoned crew has so much value when selecting a dive operation vs. those straight out of DM training with minimal experience on the local reefs.
In the ten days diving with them from the Grand, they new how many the boat could hold and if there was demand, the boat was filled. I didn't notice anything that indicated the boats were overloaded. They did keep the dive groups (DM + divers) to nine or less so there were often two dive groups on the boat several times we went out with just six divers + DM and no others on the boat than the crew.
If a boat has a safe capacity of twenty, I don't know that I would expect the op to only load the boat to eight or nine and turn folks away.
Marine park regulations call for a MAXIMUM ratio of 8 divers per Dive master - so that's also an issue if they were in fact sending out groups of 9.
Various reports also confirm that they had loaded the boat significantly over capacity. From reports, Max capacity for that boat is 10 pax + 3 crew (captain, mate, DM) - so if these various reports are true, then they were in fact over safe capacity. It's not just the weight of the divers, but the additional weight from the extra tanks and the extra equipment.
It was choppy that day, but the sea conditions were not dangerous for an experienced crew.