Same with St Thomas and some other locations. With ship dives at least you'll 100% receive a refund if it doesn't happen, receive free transportation, and generally won't take long so one could still explore the port on land. All those are wild cards with booking yourself and we were burned in Puerto Vallarta as the ship arrived an hour late due to a medical emergency; no refund or even a rain check. Specifically in St Thomas, we did JJ Shark Divers, they weren't doing transportation at the time and it was around $60/each round trip which included the taxi's "baggage fee" for the dive bag