Just did this about a month ago. We anchored behind boats that were just outside of St. Croix Marine (a bit to the north of St. Croix Marine). There used to be some moorings there the last time we did this (and the guide mentions them) but they seem to be gone now. We got a good hook set with no problems there. You can go ashore to St. Croiix Marine and registerer your dingy there (they require this if you are going to leave your dingy at their dingy dock) and it is something like $5 or $10. They also sell ice and the Golden Rail is good for breakfast and busy for all meals. Sounds like you have been there before so won't go into favorite spots/places to eat unless you would like that. Enjoy, we had a wonderful 3 nights in STX and a great sail over and back. Not sure why more people don't do it!