+1 for St. Croix
I've only dived the walls in Salt River Canyon in St. Croix, which I recall having some different topography. Have only snorkeled in Cane Bay which has, probably, the more popular wall on the island.
The pier is a great dive day or night. Not a bad diving locale for shore, wall, reef, and wrecks.
Cane Bay has one of the nicer beaches to relax, snorkel, shore dive, and enjoy their restaurant. The nicest beach (has been on some of those top 10 lists) is Turtle Beach on Buck Island. Accessible by boat, the beach has soft white sand and you can tie the boat trip in with snorkeling in the protected National Marine Park.
Pretty good restaurant choices are in Christiansted and Frederiksted (opposite sides of the island about 30-35 minutes apart)
Tours (ATV tour is fun), hiking, historical sites, botanical garden, and shopping are topside.
Have been to GC, Coz, and St. Croix several times and love the diving at all. St. Croix is not as commercialized as the other islands but probably has as much to do topside for a week as Coz (if you don't consider going to the mainland) but not as much as GC.
AS KathyV alluded to, St. Croix was hit by the hurricanes, and the 2nd one was bad. Haven't heard updates lately, but getting electricity to all parts of island was an issue. My aunt-in-law is still in the U.S. as cleanup/repairs are taking place. N2theBlue dive op is operating though. If you are interested in going, I would contact them for the current conditions on the island.