If you are staying on St. John there's Low Key and Cruz Bay Watersports. Cruz Bay runs out of the Westin and Low Key is right next to the ferry dock, you may see their boats if you arrive via Cruz Bay. There's also an operation at Caneel bay if you're staying there and one out of Coral bay if you are on the East side. I'd recommend either Low Key or Cruz Bay depending on where you are staying.
Going to St. Thomas every day is doable but with the new homeland security rules the ferry's aren't quite as easy going as they use to be. You can however take a ferry to Red Hook and then the 3 min walk to Chris Sawyer main shop, take a left off the ferry, it's on your left next to Molly Malones. If you want to try Blue Island take the Ferry to downtown Charlotte Amalie, check the schedule it doesn't leave as often and takes longer than the Red Hook ferry. Than walk about 10 min or cab over to Crown Bay Marina to Blue Island Divers.
I've worked at both Chris Sawyer Diving Center and Blue Island and can recommend them both. Blue Island dives the wrecks and reefs if the south side, as mentioned earlier the WIT Shoal II is a great dive.
If it was me I would do some dives with the operations in St John and than do the Friday Rhone trip on Chris Sawyer. Contact them to book it as especially in Feb. (high season) things can book up.
http://sawyerdive.vi/
I believe they can pick up for the Rhone trip in St. John at either Cruz Bay (near the ferry dock) or at the Westin. It's a day trip and well worth it. If Chris Sawyer is on the boat he tells an amazing, gut wrenching, edge of your seat story about the day the Rhone went down. You do two dives with lunch inbetween, a wonderful day. Don't forget your passport or Govt. Issue ID and Birth Certificate. Passport is the best option.
Emjoy your trip. If you're looking for a fun topside activity the new minature golf course just up the hill (take a cab or thumb a ride) from Cruz bay has the most magnificent views of any golf course I've ever seen (oh, and the minature golf course if fun to).
Chris