There are several dives shops located as soon as you walk off the cruise ship pier - Cane Bay Dive Shop and St. Croix Ultimate Bluewater Adventures (SCUBA). Nep2une Scuba Diving is located just around the corner from the other 2.Arriving St Croix Celebrity Equinox Jan dive shop suggestions or shore dives? Dive guides for hire?
St. Croix Ultimate Bluewater Adventures (SCUBA) and Nep2une Scuba Diving both have boats located in Frederiksted while Cane Bay has their boat located at Salt River. If you're looking for ease and convenance, I'd suggest the two shops with boats in Frederiksted as your cruise ship, the shops, and boats are all within short walking distance.
However, the diving close to Salt River where Cane Bay's boat is kept is some of the best and most dramatic on the island as it has the best wall dives available here.
Finally, if you prefer to shore dive, Sweetbottom Dive Center located on Cane Bay is the place to go when cruise ships are in and you can't dive the pier. They specialize in shore dives. Both Sweetbottom and Cane Bay Dive Shop's boat would be a short taxi ride. If you have non-divers, Cane Bay is a beautiful beach with a great restaurant/bar across the street and good snorkeling too.
I personally normally dive with St. Croix Ultimate Bluewater Adventures (SCUBA) as they have shops and boats on both ends of the island and are very well run. However, ALL the dive shops on the island have great reputations and other than weather/mechanical related issues, I've never heard a negative comment about any of them.
I try to keep up with this forum for St. Croix related questions, so if you have any more, just ask and I'll probably be able to answer within a day or so.