We negotiated a private tour with the 'salesmen' standing all along the boardwalk. They will barter. I mentioned that I am a Senior Contributor to TripAdvisor, and that, to, helped.
Our guide was half Mayan, and shared much as we rode around....but especially at the temple sites that we visited. It was 3 days before the reported end of the world (end of the Mayan calendar), and he just laughed and shook his head when we asked about it...saying "You all have made such a big deal of this...it's just that now we must merge our calendar with yours...nothing more!"....
There is a pharmacy in port that has many popular items in stock. And plenty of places to eat.