we did the BOOBS cruise out of the Albatross Marina in Cancun. All, and I mean all the women were topless, and some of the women and some men were totally nude. The trip lasted about 4 hours and drinks flowed like water. It was a blast.
We did snorkeling with the Dive Shop there at Pearl and it is NOT clothing optional. You need to wear swimsuits because you are out in public waters with other divers. It is an absolute blast though!! Really beautiful swimming in the reef.