I can't be of too much help on this, but you reminded me of a funny story from our cruise.
Immigration was late boarding in St. Thomas and we were tendering in that day, so getting off the ship was a trying experience. There was a woman in the theater who was screaming at the CM because she had to wait for a tender and she was going to be late for a private charter she had booked. She went on and on about how this was a "wish" trip for her child (who was no where to be seen), and how everything was going to be ruined for her.
I was horrified at the way she was treating the CM, and I made a point of telling him how professionally he handled the situation. The funny part was that she looked EXACTLY like Ursula. If she could have grown a few more limbs, I would have thought they were twins. We saw her several more times on board after that, and got a kick out of calling her Ursula.
I mentioned this story on the boards upon my return, and someone who was also on the cruise responded that he had witnessed the same exchange, and commented at the time that his family thought she looked like Ursula too.
If there is a point here, I guess it would be to factor in a possible delay for immigration and/or tendering into your plans if you charter something on St. Thomas. St. Maarten might be a better bet timewise.
Have a wonderful time.
Denae