I agree with ItsBruce, at least about Cayman. With 12 dives, a cruise-ship booked "cattleboat" dive is exactly the type of dive the OP should go on. The DM's know that a high percentage of their divers are newbies or resort course divers so they pick shallower locations for the dive(s) And as someone mentioned, the boat doesn't leave without you if you book through them.
Eden Rock, while convenient, was probably the worst dive we did there all week - nice people though. It's rundown and tired from the thousands of cruisers who dive there daily. Any westside dive on a "cattleboat" would be better.
If you want to shoredive instead, try setting something up with Sunset House, it's about 1/2 mile away and considerably better diving. Followed by lunch at MyBar there, it's the divers hangout. Catch a cab there though, it's too far to walk with gear.
Red Sail is the dive operator for RCL cruises -
http://www.redsailcayman.com/gc_cruise.html
Aquarium is a great shallow dive - 45' or less if you get asked. Also you could probably also do Stingray City in the afternoon and easily get back for your departure. It's called the most fun you can have diving in 15' of water.
If you do book with someone else, know that on Cayman, all the passengers are tendered off the ship, there's no cruise dock there, so factor that into your planning. But anyone who'll pick you up knows that, they do it daily.
With twelve dives, I'd stay away from drift diving in Coz. Since it's technically more challenging, especially the drops/pickups and some (not all!!) of the DM's are known for their inattentiveness. It's a dive for someone with more self-reliance in their diving.
my .02