Good points, Chris. A friend took me to STT on his speedboat this summer and I was astounded at how crowded the harbor was there. Boats everywhere! And, while this is just a guess, I'm thinking that there are a lot more boat transients on STT than STX. It's right near STJ and the BVIs and even PR which gives you lots of options and lots of neighbors.
Yes, STX has other places to sail to for the day. I just measured the circumference of our island and it is roughly 57 miles. A bit long for looping the island then dropping the hook. Instead you might sail to a secluded beach that people can't drive to and serve lunch and let them snorkel. You can set yourself apart by offering what you call a premium trip, an uncrowded boat and a real nice lunch and nice, clean snorkeling gear. Take pics of them sailing and snorkeling and offer them free online or via e-mail. Perhaps offering such a nice service would let you charge more than the competition for a day sail.
Or perhaps get into
VRBO or similar and offer the Turtle as alternative lodging. Right now, the island is dead and vacant, but I hear that in season rooms are difficult to obtain.
One of the local kayak tour companies offers a multi day trip with camping; you can offer something similar but with more upscale accomodations.
We haven't talked about STJ too much, but it's pretty much all National Park which means permits, etc. That pretty much leaves it out of the equation.
Check out this website, you'll see what types of other adventures, land and sea, are offered here already. I'm positive you can find your niche!
St. Croix, US Virgin Islands: St. Croix Information, St. Croix Tourism, Hotels on St. Croix, St. Croix Vacation Information, Caribbean Vacation
Have you explored STX using Google Earth? It's really interesting. BTW that harbor on the south shore with the big ships? You can't go anywhere near that, it's the oil refinery port.