Good day Richard,
Excellent summary of the dives and very well done with the photos of the Pier from the Cruise Ship and the Dive Boats. Yes -- diving St. Croix is really that easy.
St. Croix is famous for its diving. Waters are typically have excellent viz (always a factor of nature to change), creatures are prolific, dive site variety is diverse, and accessibility is easy.
What are the MUST SEE DIVES of St. Croix?
(1) The Frederiksted Pier -- WEST/FREDERIKSTED -- One of the Seven Jewels of the Caribbean. Also, #1 place in the Caribbean to find seahorses. Its like diving a Gothic Catherdral with pylons reaching towards the sky and the stained glass windows are replaced with a dazzling display of corals and sponges. There are still two sections of the old pier (destroyed by hurricane Hugo in 1989) with over 50 years of growth, too. This dive site will easily take several dives to explore, from the end at 90' (known as Three Amigos), to the end of the cement walkway at 42', or near shore and the old Pier remnants at 20'.
(2) Night Dive -- WEST/FREDERIKSTED -- The Frederiksted Pier. Bio-luminescence is a highlight! Moreover, the colors EXPLODE at night under your dive-light! Also, there are other creatures that come out at night... especially Octopus.
(3) The Salt River Canyon / The Wall -- NORTH SHORE -- 100 yards off shore and at ~40' deep, the Wall drops to several thousand feet deep (deepest point: ~13,500'). This brings a diversity of life that is not often seen in the world. Because this is on the north shore, conditions can be a bit chopy/bumpy. Waves can be knee high or 5'. Salt River Canyon is a boat ride to get to unless you use one of the dive companies that have a boat directly in the Salt River Marina. Salt River is a prehistoric underwater carved canyon/river/waterfall. Brilliant.
(4) The Wrecks -- WEST/FREDERIKSTED -- A: Deep Wrecks (2 @ 72-110'); B: Shallow Wrecks(3 @ 35=60'), C: Armageddon (the massive wreckage of the old Pier dropped at 90-120'). These are all quite close to each other.
(5) The Reefs -- WEST/FREDERIKSTED -- Calm waters, extended dives, and water features that can have the dive be 30-55', or 55-120'. There's a ridge on the west where dives can be either deep or shallow. The waters are almost always calm because this is on the wind-protected side of the island. The cover-story of a magazine was divesite: The Swirling Reef of Death. You'll have to ask for the funny story from the captain. There are over 40 divesites for both shallow and deep dives that'll keep you delighted. Very easy to book a 3-5 day package and see a lot! HUGE beaches for the non-divers, stand up paddleboarding, and the charming laid-back rest you've been looking for.
Please note: When Cruise Ships are in port, the US Dept of Homeland Security does not permit diving on The Pier (until after it departs).
Loads of options for dive companies that will rent gear, guide dives at the shore, or take you on a boat dive. TripAdvisor.com is a great resource to find out more. Lots of parking 100 steps away from The Pier.
Enjoy your travels and Enjoy St. Croix!