Apparently you can dive Buck Island on Wednesday afternoons. And if you have 4 people that are interested you may be able to arrange a private trip on a different day:
https://www.stcroixscuba.com/page/scuba_dive_buck_island
SCUBA DIVE BUCK ISLAND The Only Underwater National Monument In The United States
Buck Island’s Barrier Reef's underwater scene taxes human perceptions with the abundant variety of shape, pattern, color, texture, and movement. It’s barrier reef ranks among the Caribbean’s best. It’s thick branching elkhorn corals push their sheer mass to 30-foot heights. Like fortress walls corals rise off the sea floor and dominate the underwater world.
Through an exclusive arrangement with Caribbean Sea Adventures, we provide scuba diving to Buck Island every Wednesday afternoon. Other times may be available for at least 4 divers.
Check out more information on St. Croix's Buck Island here:
https://www.nps.gov/buis/index.htm
Buck Island isn't just for snorkelers.
Two thirds of the island is surrounded by an elkhorn coral barrier reef, providing an ecosystem for hundreds of species of marine life.
https://caribbeanseaadventures.com/tour/buck-island-tours/
DIVE BUCK ISLAND REEF NATIONAL MONUMENT
Caribbean Sea Adventures is the sole provider of scuba diving services at Buck Island National Monument. We have teamed up with the experts of SCUBA - St. Croix Ultimate Bluewater Adventures known to be "the friendliest dive shop on earth." Together we provide the ideal diving experience. A white sandy bottom at just 40 feet yields to monuments of coral built like castles in the sky. Perfect for beginners, the seasoned pro looking for a new site, or even one last shallow dive before you head back to the real world.
Price: $100 per guest, $0 Tiny Humans, -10% mil/sr discount
Guests: 19 Maximum
Duration: 3 Hours
When: Every Wednesday at 2:00pm
Meeting Place: SCUBA 81 Queen Cross St, Christiansted, St. Croix, 00820 – Please arrive 30 minutes before scheduled time
Included: Tank, BCD, regulator, weights, fins, mask, dive master