The boat was the 3 Hermanos taking folks down to Punta Sur from at least the Allegro and Grand. I wasn't on it but was diving with several folks who were and my afternoon dive was late as a result of the other ProDive boat going down to get the folks and gear.
From what I was told from those in the water waiting to get on, the boat was backing to get divers into the boat when water came up over the transom. I gathered the captain tried to go forward but kept taking on water until it sank. I heard it went down in deep water but haven't had any real confirmation (taking on water, the captain should have headed toward shore as much as he could.)
One mate was injured (don't know the extent but didn't sound serious.) A lot of luck involved as the boat had a covered / enclosed forward area including a cabin with head. Apparently a diver on the boat was rescued through the plexiglass windows that f are on the side of the forward (not cabin) area.
My first time here in Cozumel and was kind of surprised how much these boats back down to pick up the divers from the stern. The weather yesterday was rougher than it had been with wind from the north at about 15 kts. Seas were maybe a meter or so, or less, in the area I was diving all day. The seas combined with the backing maneuver sounded like it sunk the boat.
Again, I wasn't there but heard the story from those who were. I didn't see anything to indicate the boat was overloaded either when I was on it or the number of divers taken on the trip.