Greetings Jandee and thank you for asking about St. Croix. I invite you to take a look at the images posted on Facebook to give you an idea of what you might see.
I know I've posted this before, but it holds true:
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. Late Feb-March is a great time to hear (and sometimes see) whales. Dolphins are abound and playful when encountered. Feb/March water temp is 77-78'F. Sept/Oct water temp is 85'F. The wind/waves are East-to-West, thus the west end (Frederiksted side) of the island is in the shadow/lee and is typically flat waters like a lake with about 30+ well known and 20-30 lesser known divesites -- Most popular are The Frederiksted Pier, three different divesites for Wrecks, and the reef (exemplified by famous Swirling Reef of Death -- featured in a dive magazine cover page years ago). The north shore has a bit more for waves and features the Salt River Canyon and the Wall.
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. :cool2:
(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!