You will certainly not be alone in taking a cab from the ship to Maegens Bay (and back).
Do keep in mind that many cabs in St Thomas are large vans or jitney trucks seating as many a 10 or 12. When you get in the cab (you will be directed to one by a dispatcher
at the port) he may want to wait a few minutes until he has a few more passengers.
Be insistant if he waits longer than you think is reasonable. Fare to Maegens is a set amount, about $ 8 per person . You will also have to pay the entrance fee to Maegens in cash , which I believe is still $3 per person.
Cabs to Paradise Island are plentiful and are only a few dollars. Another alternative is water taxi, but you have to walk up the hill several blocks from the pier.
As for Nassau: When you get off the pier, there are water taxis. They charge $3.00 each way to Atlantis. You really get the local flavor taking these boats. It was fun. A local acted as a guide on the ride over. We all tipped him a dollar. When you get to Atlantis, it is a bit of a walk to the casino. Enjoyable though. When we came back to the boat to return to the ship, the boats were lined up and ready to take us back. We even had a Bahama Mama on the dock on the Atlantis side. It was great. We firsdt inquired about a taxi to the casino, expensive.
St Thomas. a party of four f us took an open caravan for a two hour tour. We were charged $20 each. It was fabuous. It lasted about two hours, all the way to the top of the mountains and back. Lasted about two hours. Some great photo shots. Saw that bankrupt ship in the water, felt so bad for those people,