Please, never hold your breath on scuba. It's a bad habit to even consider.
I edited previous post - thanks for pointing that out. I had meant (thought it was obvious) before going underwater. Many new divers have too much weight to make going down easier, then that extra weight makes becoming neutral so much more difficult.
I use just enough weights so that when I full exhale on the surface, I start going down, then breathe normally to the bottom or dive limit. Then remaining neutral is so much easier when below 10' or more, compared to having an extra 5 lbs or more.
As for all the other advice - I think us older more mature folk have to think back to when we were 21... I know I would not have wanted to waste precious time in buses and such. Just getting off Cozumel to Playa is an exercise in patience.
I fully expect these boys to go down to Mexico more than once.
If they split between two or three places, won't do much cost-wise, but will cost them time-wise relocating. If they are there for 7 nights, and the day they arrive they won't dive, and the last day & night - they should not dive - that leaves 5 days of two to four tanks.
Are there enough, different location, dives in the Cozumel area to justify 10 - 16 dives in five days?
If not, then they should live in two different spots...