I'm not a SCUBA diver, but I've been on snorkeling excursions in St. Thomas and St. Martin. They've all been from a boat, with non-snorkeling companions enjoying the boat during the swimming. Hopefully, someone else who does SCUBA can help you with this one.
Yes, you can get "intro to scuba" excursions on that itinerary. I believe it was in St Thomas, but it could have been both St Thomas and St Martin. Check into the excursions list and look for the intro classes. They take you out off a beach (maybe 25 feet of water??) and they give you a how to class before hand. They claim the dive will count towards a certification too. I took my eldest daughter and we both had a great time.
As far as dive huts... On the Mariner they have a dive shop and dive pros who do the classes and diving excursions, so you should be able to talk to them also.
Magens Bay - The bottom is flat and sandy, this is a place for sun and swim rather than snorkeling.
Coki Beach - next to Coral World is the island's best-known snorkeling spot.
Morning Star Beach - snorkeling is good near the rocks when the current doesn't affect visibility.
Sapphire Beach - snorkeling is excellent at the reef to the right, or east near Pettyklip Point. Also good for windsurfing.
Secret Harbor - superb snorkeling to the left of cove near the rocks.
140-acre island. Previously known as Little Stirrup Cay. Snorkeling is good around sunken airplane and replica of Blackbeard's ship, Queen Anne's Revenge. Activities and games ashore. Claims to have the largest Aqua Park in the Caribbean. Scuba diving, snorkeling, jet skiing, kayaking, parasailing, hiking, volleybal, shopping.
St. Martin/St. Maarten
Not known as a dive destination. For snorkelers, Orient Bay, Caye Verte, Ilet Pinel, and Flat Island protected as regional underwater nature reserve. Offers half-day snorkeling trips.
JV, I don't know if you can contact the on board dive shop prior to sailing. But they are open as soon as you get on the ship, so you can check thing out early. Or like Penny or Suzy pointed out, you can contact shops directly via the internet or phone. If this is your first cruise, while it may be more expensive, I would suggest sticking to the ship booked excursions. Thise you can book before hand. If you are certified, make sure you bring your dive card as the experienced dive tours will require that you have it with you.
But if you are adventurous I certainly wouldn't claim you can't go book things on your own. If you book through the ship (especially for the beginner dives) the ship will obviously provide the scuba gear. You can also bring your own stuff (provided you can get it to the ship) For the dives booked on your own you would have to inquire with them at booking on if they can provide gear.