Here's a tip, both islands are in the USVI's...
- the BVI's are the next group of Islands over (Tortola, Jost Van Dyke, Virgin Gorda, Anegada)
Besides Admiralty, Blue Island Divers is another option - they'll pick you up off the pier by boat. If your ship docks at Crown Point instead of WICO/Havensight, they're walking distance in the same marina.
http://www.blueislanddivers.com/
Don't got with the St. Thomas cruise operator - there's a thread here about how bad they are. Something with a "S" in the name IIRC.
There's no cruise dock on St. John so everyone is tendered in off the ships that stop there also. Cruz Bay Watersports is an option as they have a location downtown. I don't know anything about them.
http://www.cruzbaywatersports.com/scuba-diving
Red Hook Diving is good but they're about 30-40 minutes away in Red Hook (go figure) so depending on your arrival time in St. Thomas you may not be able to get there in time for their 9AM morning departure. If you're looking for a low stress dive experience, the logistics of getting there may not be it. There's going to be a fleet of cabs at the port but many are shared vans so they'll try to fill them before departing. If your schedule is tight, avoid them - they'll drop pax along the way to Red Hook which is about the furthest stop.
http://redhookdivecenter.com/
From either location Cow/Calf Rocks is a shallow "mellow" dive and one of the better known St. Thomas dives. A lot of the Cays diving further east is very protected also - Cruz Bay should do some of it - it's a little far for the Charlotte Amalie boats since they like to get you back in time to look around town a little.
