I've been to Provo and St. John once each. Since you've already been to T&C twice, I would recommend you try St. John.
As for the underwater delights, Provo was better. (But I HATED the long boat rides.) St. John was plenty good, though. Not deep, but good reefs and plenty of sea life.
As for topside, St. John is far and away the place to be. It's a beautiful island. Small with some nice secluded beaches. Some nice hikes if you like that. The tiny town of Cruz Bay has some really good food and bars. Walk around with a drink in your hand just like Las Vegas. General prices on St. John aren't exactly cheap, but it's a lot cheaper than T&C. There are only 2 resort-type places on the island, The Westin and Caneel Bay. I stayed at the Westin and really liked it. Beautiful. The beach, however, is mearly adequate in that it is manmade. Caneel Bay has several great beaches, but the accomodations didn't interest me. There are some small hotels without a beach plus rental houses, etc. but I don't know much about them. The Westin (and I believe Caneel Bay) has their own ferry service to St. Thomas which is very convenient. I came down with a cold during our trip and that put an end to scuba. So we took a day sail/snorkle trip to Jost van Dyke BVI. That was super fun. Some of the best snorkling my wife and I have experienced (I belive the snokle site was actually in the USVI). Then a sail to Foxy's in JVD for lunch. Then several hours at a very beautiful beach on JVD. Nice day even though I was quite ill from my cold.
Send me a PM if you want more info.