We booked a Snuba excursion, it was cancelled due to water conditions. We rebooked a Chakanaab day pass, and that was also cancelled due to water conditions, which we didn't find out about until we were on shore. Luckily we were given a refund. Don't bother hanging around the port, unless you like paying too much money for overpriced crap. Grab a taxi and head to the other side of the island to enjoy the beach. We spent a very nice afternoon at Playa Chen Rio, had lunch at the restaurant, swam a little, then just relaxed on the hammocks next door. Don't bother going into town, either...it's overflowing with people who want to take your money in exchange for complete crap.
This SHOULD be the longer port. Didn't bother with an excursion, as we were only in port for four and a half hours. Literally everything in Old Town is walkable from the port. We went to South Beach, and my friend took a jet ski tour with Barefoot Billy's while I walked up and down Duval shopping. Wish we could have stayed the whole day.