Hey Greg. On the Norwegian Dawn's itinerary in August, there is only scuba diving in one port. Scuba diving is offered from the ship in Nassau, Bahamas and is offered by a company called Divers Haven. The dives change each time, depending on conditions. The dives are either at a shipwreck, local reef, or something called the Paradise Caves. This is an operation that is on Paradise Island in Nassau. Here is their contact info, but it looks like they have no website as of yet.
Divers Haven
Nassau/Paradise Island, Bahamas
Phone - (242) 363-3333
Fax - (242) 363-4401
I also found this, not sure which is current but you can try both.
Divers Haven
P.O. Box N-1658
Nassau, Bahamas
Phone - (242) 394-8960/8961
Fax - (242) 393-3695
This itinerary also goes to Port Canaveral and Great Stirrup Caye, NCL's privately owned island. There is good snorkeling offered on GSC, guided by NCL Instructors that live on the island full time. I have not been there yet but have heard it's excellent. Royal Caribbean owns an island nearby and I have heard of the instructors taking a boat over there on off days and diving. Maybe you could inquire about that at the Shore Excursions desk when you board the ship in August. I hope this helps. Let me know if I can be of any other assistance. If not, then come see me on the Pride of Aloha sometime. I should be on it soon and once I get there, I'll be there for a good while, unless it's vacation time of course. Because living and working on a cruise ship and guiding snorkeling trips full time isn't vacation enough.

Good luck and let me know how it goes!