We did that exact itinerary last year and dove in both Tortola and St. Thomas. I would strongly recommend that you dive the RMS Rhone in Tortola. When you consider that the wreck is well over 100 years old, you'll marvel at the condition, particularly the bow which is still intact (partially). The prop is HUGE and it is easy to distinguish many of the different parts of the ship (the boiler, the masts, etc).
In St. Thomas we also dove the Navy Barges and a reef, both of which were really great. The Navy Barges were fun to swim through and explore, and the reef was in excellent condition with lots of coral and marine life.
We didn't dive St. Maarten because it isn't really known to be a good dive area. There's often strong ocean action and the diving can be dicey, so we spent the day on Orient Beach instead. I don't believe you can dive in San Juan either...at least I couldn't find a way in such a short time.