Can't answer the butterfly question, but will relay our Coral World experience.
We left the ship at 8:30 to take a cab. We were instructed by the dispatcher to get into a certain vehicle. There were a bunch of cabs going downtown, but only one going to the Coral World vicinity. The cost was $7 per person and DD could not sit on my lap.
Also, we had to wait until it completely filled up. We waited and waited and waited while the driver hustled people to get on. Finally, at 9:15 we were off. It had been so long, the driver forgot where we were going and started taking us on the St. THomas tour and wanted to charge us for the two stop we saw before reminding her we wanted to go to Coral World.
By the time we got to Coral World, it was close to 10:00 and we were pretty steamed. For the three of us, admission cost $48. I think it was $18 per adult and $12 for children over 3. Honestly, I don't think it was worth the money. I felt the facilities were poorly kept up. The "hands-on" section was really lame. I've seen guppies that looked fiercer than the sharks. I was really disappointed and it didn't really keep DD's interest. She was more interested in seeing the Sea Trekkers than anything else. And, by the way, don't bother with that. Everyone that I saw do it looked looked miserable. And it looked somewhat dangerous. People were slipping on the the ocean floor and did not look comfortable at all.
One of the reasons we wanted to to Coral World on our own was so we could spend some time at Coki beach. But, because of how long it took to get there, we didn't have time to do Coki beach and go shopping downtown.
So, from Coral World, we took a cab (again, $7/person) to downtown. We shopped for a bit and took a cab back to the ship ($3/person).
All in all, because of trying to do Coral World on our own, I'd say our visit to St. Thomas was a bust. Next time, I'll pay a little more to do a sanctioned excursion, just to avoid the frustration.