Just pointing out that St. John is very easy to do on your own.
We exited the ship got on the waiting taxi to the ferry, which we made with no problems.
When we docked, we walked one block to rent snorkel gear, then went back to the ferry dock and got a taxi to Trunks Bay.
Snorkeled and relaxed on the beach.
When we had felt like we had experienced this, we walked back out front, and took a taxi to Cinnamon bay. Swam again.
Cinnamon bay had a Sugar Plantation ruin across the street with a nature trail.
When we were done, we took a taxi back to the dock. We could have hoped on the ferry right then, but had a late lunch overlooking the bay. An hour later we got on the next ferry, taxis were waiting at St. Thomas to take us to the ship.
The only thing I would recommend is work out what return ferry you need to be on, and then make sure you on are the one an hour prior.
We had a fantastic day, much cheaper than the official excursion, and our table mates were jealous as all they did was drive around the island and look at the water.