We booked Nachi Cocum online. It's an all inclusive resort about ten miles from the ship that limits it's customers to 100 per day. Well worth the money (around $100 total for two) and includes all drinks and food. Super nice place, very friendly staff. Would go back most definitely.
We did this port on our own. Took a cab to town and booked a snorkel trip for a couple of hours. Did some shopping and Senor Frogs too. The public beach is beautiful and although seeing police and military with machine guns can be disconcerting, we never felt unsafe. Got some great shopping deals and just had a fun time.