Any recommendations of which island and good dive operator?
Can't comment on the other islands but here's a report on St Croix. If you are doing only a few dives my very strong recommendation is to dive the Fredrikstead pier. Very easy to do on your own.
Cane Bay
This is a beautiful beach with the Cane Bay Dive shop directly across the road. We were able to rent tanks and weights for $15 for 24 hrs and refills were $8. We did these dives without guides and had no problems. The swim out to the drop off is rather long (about 150 yards) and the current always seemed to work against us. You can go down about 30 yards early and explore some coral outcroppings along the way. The best entry is right along the boat ramp since it's sandy whereas there are rocks to either side. Swim out to the buoy just left (west) of the boat ramp then continue directly north to the edge of the wall (or drop down first). The top of the wall starts at about 80 feet and the coral is is pretty good condition. Visibility was ok at about 50 - 60 feet. Hurricane Sandy had passed by about a week earlier and this probably still had an effect on the visibility. The wall heading west was better than to the east where the sandy bottom extends further to the north making the top of the wall deeper. There were lots of fish and we saw a reef shark on one dive. We did all the north side shore dives at Cane Bay because the other dives were very isolated and it wasn't obvious to us where the entrances were. Ask at the dive shop for directions to the giant seahorse (3 ft long - no lie).
Salt River
We took two dives with Anchor Dive Center (East Wall and Shark Shanty). The boat and crew were good and the boat ride was only 5 minutes to these sites. That was good since the water was a bit rough (combination of the normal condition for that time of year and the storm surge according to the captain). Our DM was "Whiz" who was great. On the first dive we had three reef sharks hanging nearby and also found a nurse shark under a ledge. Nice dive. The second dive was only ok, nothing special.
Fredrikstead pier
We used N2 the Blue to rent tanks and weights but did the dives on our own. Their shop is perfectly located for diving the pier. It's only about 100 feet from the start of the pier and they also give you a cart to haul your gear. The carts have locks so you can leave them on the pier during your dive. We walked out to the wooden pier to the south side and jumped in to 8 feet of water and snorkeled to the start of the main concrete pier. There were thousands of small fish schooling along the way and swimming through them was like going through a snowstorm. Be sure to check out the remains of the old pier (destroyed during a storm) that stick out of the water to the west of the entry point as this has had more time to grow coral and is quite nice. The main pier has coral growth on all the support columns and rocks/debris on the sand where you'll find a great variety of fish/critters. Also be sure to check out the sand and rubble to the north side of the pier heading back toward shore since there were several things to see including two octopi, flounder, stingray, webbed burr fish etc. We saw many interesting creatures under the pier including lobsters, crabs, two large green morays, two seahorses, two more octopii, sharptailed eel, a second webbed burrfish, large porcupine fish, a really tiny juvi french angelfish (1/2"), juvi trunkfish (looks like a die), juvi hardhats, tons of very large arrow crabs etc, etc. This was a super divesite. Four octopi in one dive and this during daylight!!!! As we did additional dives we would snorkel further out along the pier before starting the dive so we could explore more new areas. The max depth we hit was about 50 feet (in some dropoffs to the south side) and our dives were 85+ minutes long. Visibility was about 40 feet at the start and about 80 feet as you proceeded along the pier. There is a place where one could jump in a lot closer to the start of the concrete pier but the gate leading to that area was closed while we were there so you had to jump in at he wooden pier or enter at the small sandy section of beach. The exit was by climbing up the large rocks at the east end of the wooden pier (or via the beach). The rocky exit was easy as you could sit back on the dock while standing on the large flat rock and remove your tank before climbing up onto the dock. BE SURE TO DIVE THE PIER. This was one of my favorite divesites ever.
Shore diving was tough due to distance from shore and lack of parking.
