Snorkel / free dive only.
This has been going on for 10 years, but only recently has the world found out about it. About 7 years ago one of the dive magazines published a front page cover story about swimming with the sail fish at Isla Mujeres. That coincided with about the first time I visited the island. At that time I asked everyone I could find there about it and nobody knew anything about doing it. The back ground is Isla Mujeres is one of the top sailfish fishing places in the world, the season is roughly Dec-March, and every year for decades boats have come down from the US to stay at Isla for the winter sail fish season. It started as rum buddies and has no become, send your boat and crew and the owner flies down every week end and takes the boat out.
Anyways, a pro photographer scuba journalist got the idea, hey, sail fish, bait ball, me got camera you take me out and I'm getting in the water and see what happens and that's how the first article broke. Like I said when I first went there nobody had any idea about this and certainly there were no commercial ventures offering it to tourists. Well actually there was one, they weren't based there and it was more of an expidtion and it was about $10,000 a person.
Very slowly this is changing, more pros did photo essays, more magazines featured it, a couple of people on the island figured this out that this might make some money and now this new tourist industry is just on the brink of developing into something. There are still only one or two outfits that really know what they are doing in regard to this and then a couple fringe people who do all kinds of things there and are throwing their hat into the ring if you dig them up and offer them some money, and there are a couple of photographer specific USA based operations that put together a week long photo excursion ($4000-$5000 I think). People like the Keen group are more deep sea fishing companies and I'm sure they are still testing the waters, the season to swim with them is the same season to fish for them and after all every day they take one of their boats out of the very expensive deep sea fishing days they have to make up the money, there are a lot more single and small groups that will fork over $2500 for the day to fish then there are who will fork that over to swim with sail fish, so they probably need a lot of people in the water to make it affordable and I think that is the issue right now, because not many people have even heard of this.
But it's starting and its not developed like the whale sharks thing, it might be someday, but if you want in on something really unique and wild and just developing nows the time.