Been to all three ports several times. Coz and GC are easy to do on your own; OR - I wouldn't, except to go to Island Village.

In GC the "must do" is swimming with the stingrays, and you can book this on your own. Captain Marvin and Nativeway are both good. The latter has a great trip that also goes to Rum Point, but it may run very close getting back to port to catch the tender, so you'd have to consider that. You can also snorkel at Eden Roc, which is a five-minute walk to the right [as you face the town], maybe even while you wait to go to your stingray tour.

In Coz you can catch a cab south to Chankanaab Park [see their website] or what used to be called Dzul Ha [can't remember what the name is now].

Don't think the snorkeling in OR is that great. If you just want to relax without being hounded, you can walk five minutes to Island Village [see website], where they have some shops and a Margaritaville with pool, hot tub and small beach. The locals will hound you before you get to IV, but then you'll be left alone. IV is to the right as you walk out of the port area.

One thing you must remember if you go out on your own is that you need to allow plenty of time to get back to the ship, because they will leave without you. [Especially important in GC because you have to tender.] Be sure you take the info about the contact people in port that's listed in your daily activity sheet and you may want to take your passport and credit card just in case you do miss the boat. There are two schools of thought re taking your passport ashore - you decide.

For lots of great info on all your ports and other aspects of cruising, check out CruiseCritic.com. Click on "Boards", then go to your area of interest.

Have a great time!