We just got back today from our cruise and did take a cab from Punta Langosta pier to Chankanaab Park to do some snorkeling. A cab (regular car) that can transport no more than 4 passengers costs $10.00. If you have more they take you in a van for $15.00. Very easy to get a cab from the pier and back and then back from the park. Ride is only 20 minutes. Park is beautiful and has everything you need such as clean bathrooms, restaurant, etc.
You are smart to do this trip on your own as many people said it was much cheaper to do this way than through the cruise excursion. We certainly saved a lot of money doing the snorkeling on our own with the cab.