ScubaSteve
Wow.....what a DB
Booking your dive through the dive shop instead of through the cruise ship is much less expensive, as are any shore excursions. In addition to suggestions here, try cruisecritic.com or tripadvisor.com for reviews and recommendations of dive shops to go through.
To the OP: As will be pointed out many times, booking a dive through the cruise line is likely to come with a premium. My experience is limited and dated about 10 years ago. I have been on one cruise (will never again) and dove at 3 ports. There was one single reason for my choosing to book through the cruise line rather than direct. If you ever run into any issues that delay your return, the cruiseline will make all arrangements to get you back to the ship (even if that means at the next port of call). If you book on your own and miss the departure of the ship, then you are SOL. Delays can be due to odd currents on the dive, boat issues, people issues......or anything really. When looking at the cost of the friggin cruise.......the premium on the dives was no more than a mitt full of celebratory drinks on the cruise. I gladly accepted the premium as "insurance". YMMV (I would suggest confirming the cruise lines policies regarding missed ships for yourself as things can change at any time.