Isla Mujeres is way up by Cancun and purely from the location given of the other two, it's fishermen.
You want to learn something interesting and maybe gain a bit of respect for that fresh seafood that you eat - go down by the people ferry 6:30 or so in the morning - watch what those guys load in the boats, the equipment and the boats themselves. Talk to them about how they work - I think you'd be shocked and maybe have a better understanding on how this guys get misplaced for a bit.
Some of the boat captains and mates have a background in fishing, ask them about it - one day, I'm gonna strap a computer to Gato's ankle to see what he really does. I've heard that they go down, no BC, just weight belt, and that the captain is supposed to drop them another bottle at predetermined times - they'll dive two or three bottles before surfacing.