I just got back from my first St. Croix trip on Sunday. I was there for six days, staying at an Airbnb in Frederiksted. We wanted to be walking distance to dinner/drinks options (as to not have to drive on the left after drinking!) and close to the pier for diving.
Yes, I agree with the above posters: the airport was packed, the floor and ceiling are currently torn out, there were three flights' worth of people waiting, a long line for the snack bar, and no AC. But that shouldn't really be a deterrent to travel - just a heads up to dress cool, bring something to do, (especially something that may calm your nerves.), and don't assume you will get an outlet to charge devices. It's not the worst airport experience I've had. On the plus side, the women's bathroom was better than expected. If you bring your passport instead of just a drivers license, you may get to bypass to a shorter line by security. I thought that was a great deal, until I saw the area I'd be waiting in once I got through security anyhow.
We rented a small SUV from Centerline, and I would use them again. Their prices were comparable to others, and we had a good experience. When you arrive, if you have a checked bag to pick up but there's no line at the rental window, you might consider going to the rental window first. I got my bag first and then ended up behind quite a few people. Our Airbnb host said that some car rental places charge $150 for sand in the vehicle. We did vacuum ours out just in case, but they didn't seem to be overly picky and drop off was fast. They shuttled us to the airport after drop off.
Driving on the left wasn't an issue for me. I have quite a bit of experience in Grand Cayman, but it was a long time ago. This was a lot easier than that with significantly less traffic than by Grand Cayman's Seven Mile Beach.
Do bring a face mask (like, the covid type, not just the diving type). As of Sunday, stores and restaurants were still requiring masks. For restaurants, you need to wear it when walking around but not once you are seated.
We used Adventures in Diving in Frederiksted. Like a couple other shops right by the pier, they were very conveniently located for walking to/from the pier. Because we stayed just a few blocks away, we could easily go back to our place for lunch in between dives, etc. We ONLY did shore diving, and decided to stick to the easy sites, so stuck to the pier and Cane Bay. A few others were mentioned to us, but we were enjoying these enough that we stuck to them. Every tank we received was over 3000 psi and they also double-checked them before sending us off with them. They have a nice bench with tank loops in the courtyard for gearing up.
We got lots of different dives out of the pier, and could continue to get more. Unfortunately, the vis was terrible the last day - fortunately, we had seen it when it was good, but if that was our first dive there I'm not sure we would have repeated it. We entered both from the small wall to the left of the pier and also giant striding off the platform on the right (north) side of the pier by the "Fish Sustainably" mural. We would check out the pillars, the wall of the pier, the rubble, the "dolphins" (old tie-ups) and the rocks.
Speaking of the pier, we enjoyed walking it at night, but it was closed the last couple nights, so maybe take advantage of it early in your trip in case you can't get out there later. We got a kick out of watching people fish without fishing poles (even though they sometimes had poles lying right next to them!)
We ate at Nacho and Louie's (twice), Lone Dog Pub (or something like that), Tap Deck, the restaurant at Sandcastle resort, the restaurant across from Cane Bay, and I can't remember where else. (Lunch was typically PB&J in our rental.)
We did have to take a Covid test and upload it to the "USVI travel portal" before the trip. Apparently it's usually pretty fast to get approval after uploading, but mine took more than 48 hours to come back as approved (AFTER uploading my results) so I wouldn't save it until the last minute.