I spent a week last July in St. Croix and had a really fabulous time with a buddy of mine! Granted it wasn't a dive vacation (but I did get him to try scuba twice and he loved it and the scenery during our shore dives).
I've been to plenty of touristy places in the Caribbean and I was getting burned out on all the mass crowds of typical tourists everywhere, packing everything, making every place homogenized and I found St. Croix to be just what I needed to love the Caribbean again.
It's not a very touristed island anymore as the cruise ships pulled out of there some years ago, so the accommodations aren't super deluxe, but definitely clean and comfortable (like a couple walls had cracks in the concrete, no big deal to me, but some people don't like that). The people were VERY friendly, both the transplanted Americans that live there and the native locals.
I think the only places we saw other tourists were our hotel (Carambola) and the Cruzan rum distillery for the tour (and free all you can drink sampling!)
There are plenty of good restaurants all over the island. We made a habit of eating lunch and dinner in diff restaurants every time and it was GREAT! The food was really good in just about every spot (except one) and ranged from a typical sit-down dinner to a burger shack on the beach overlooking the surf. Prices were fair and the couple places that were expensive definitely were worth it with the quality of the meals and the presentation.
Some people have complained on the 'net about St. Croix not feeling safe, but I don't know what they are talking about.. We drove everywhere there at all hours of the day / night, including more modest local neighborhoods and we NEVER felt unsafe.
The beaches are basically free on the island and we always kept our snorkel gear and swimsuits in the trunk so we could literally drive around and pull over when we found a nice stretch of beach to go snorkeling and then have a good strong drink at the local beach bar.
I only dove off Cane Bay where its a shore entry with a wall maybe 200 ft off the beach but it was as good as the diving I've been exposed in the Bahamas. I currently have plans to spend 3.5 days there again this July and have already booked 5 boat dives with 2 different operations on opposite sides of the island. I can't wait!
I hate the over homogenization of many places in the Caribbean. Same crappy souvenir shacks/stands/stores selling the same crappy merchandise. Same crappy tours available on each island. Same crappy tourists thinking "wow. this is how the locals live.".
For me, St. Croix is what the Caribbean should feel like. Kinda like those silly Corona commercials where all you have is the blue sky, beautiful water, and people relaxing like time doesn't matter. I hope to go visit there for years to come!