Skip the tour companies who want to take you to Cancun's reef for more then $45USD, and rent a car or take a cab even, to PM. Go to the main dock and ask for Charlie Brown. This dude will take you to the reef and dive with you, showing you things you will not see/find on your own. He showed us 3 stingrays, each one bigger than the last, with the 3rd one being absolutely enormous! Buy and underwater camera and get it all on film. Base price for this in PM is about $22USD. But tip the guy if they do a good job, which Charlie did. Save money by doing it on your own and have an even BETTER time.