I had a great experience with Down Under Watersports in May. It's a fairly new operation that only takes out 6 divers at a time. They went to some great dive sites and we really enjoyed diving with Bill and Cindy. I also dove with Low Key and felt it was more a cattle boat experience, so we decided to try Down Under and we are glad we did.
Also, we went out one day with Cruz Bay Watersports and did the wreck of the Rhone in the British Virgin Islands which was a great dive.
I really enjoyed the diving there more than I thought I would, especially since Bill took us to some really great sites where the reef was in good condition and had plenty of life. Down Under was also the only operator that allowed me to stay down until I was at 500 psi. The other operators are on a very strict schedule but Down Under was very relaxed. You may also want to check out a post that is still active titled St John USV. It has several pages of good information. If you have any other questions feel free to ask. Good luck and enjoy St. John, we sure did!