If the "Spanish cannons" that are out in the bay today are the same ones that were out there in the early 1980s, they are not cannons, but two 16th century wrought-iron (not cast iron) versos. These two breech-loading, rail-mounted guns were taken to Akumal Bay in 1960 and 1961 by members of CEDAM from the Bahia Mujeres Wreck that lies between Isla Mujeres and Cancun. In all, the Mexican dive club "rescued" 1 falconete, I bombardeta, 2 versos, and an anchor from that site.
I remember snorkeling out to the versos in Akumal in 1984 with another archaeologist to measure them and almost drowning because I was laughing so hard at the sight of him being harassed mercilessly by a damselfish who objected to us picking up the gun (where it was living, at the time) and walking it to shallower water. Where it lay in Akumal bay originally, was just deep enough that our snorkels barely broke the surface of the water as we struggled to carry the heavy piece towards shore. As I would start to laugh, I'd get water in my snorkel and we'd have to set the thing down. The damsel fish would swim back into the breech, only to come back out pissed off and ready to fight every time we picked it back up and started walking with it again.
Later, we mapped and surveyed the original site of the Bahia Mujeres wreck in a joint project with INAH and INA. I remember getting off a Continental flight in Cancun, changing clothes in the taxi, jumping aboard a small speedboat that was waiting to take me to Isla Mujeres, where I boarded a Mexican Navy helicopter to fly out over the site and mark it with buoys as we hovered over it, then flying back to the dock to meet up with the rest of the crew to go dive the site. The plan went straight down hill from there. The microwave tech who we hired to man the positioning system we were using plugged a 110 V unit into a 220 V outlet and fried the thing, our boat swamped and sank over the site, and my wife was injured and needed to be flown back to the states, where to her dismay and chagrin, TV cameras rerecorded her deplaning the flight in a wheelchair.