Hi Vance, welcome aboard! Congratulations on booking your magical Fantasy cruise with Disney! What an amazing way to begin your summer!
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Full Day Caribbean Sail & Snorkel to Turtle Cove and Honeymoon Beach is sure to be a wonderful adventure for your family. As with all of the long all-day Port Adventures, Disney will ensure you are back with plenty of time to board the ship before it departs. That being said, at times there are long lines in the ports to re-board, so I wouldn't recommend wandering far from the port once your tour drops you back off. Certainly you should have a bit of time to wander the port area shops before heading back onto the ship, but most likely you won't have too much time.
The all day tours can vary in exactly how long they are gone depending on a number of other factors, so you may find yourself back with plenty of time to explore, but most likely you'll only have a bit of time before you'd really need to board.
I hope that helps as you plan, Vance. You will certainly have an amazing adventure, and I guarantee that when you stop in St. Maarten, you will have plenty of similar shopping spots, so even if you don't have much time to explore the St. Thomas shops, you won't miss out altogether in the Eastern Caribbean!
Please let me know if you have more questions, as I'm very happy to help!