A little help with the logistics on Grand Cayman.
Eden Rock is about 100 yds. from the cruise tender dock. Fosters is slightly farther. Eden Rock is one dive, Devils Grotto is also there to the south. DG is slightly overhead obstructed so stay in the passages where you can see clear "outs" at either end and possibly in the middle. Neither site is going to be much over 40'.
Scuba Dive Cayman with Eden Rock Diving Center
Fosters or Red Sail are the Cruise operators depending on your cruise line. Both have a lot of big boats but as newer divers that's not always a bad thing. They'll still provide a DM for every 8 divers per law and may go to some mid-range sites more suited to your experience level. As someone else mentioned, Wall to Wall tends to do more experienced deep wall dives if possible.
Another option the other way is Wall to Wall Diving or Deep Blue Divers. They work out of the Lobster Pot Dive Center - a short walk north of the port.
Scuba Diving in Grand Cayman with Lobster Pot Dive Center | There's also a shore dive there and Wall to Wall will provide a free tank for it after you boat dive with them. They also have a cruise dive program so their 7:30AM departure isn't an issue.
http://www.lobsterpotdivecenter.com/cruise-ship-visitors/
Sunset House as mentioned would be another easy option. A few minute cab ride south from the port. They have both a dock entry and the Salt Water Pool for a shore entry. Decent diving - better than Eden Rock but not the best on GC. The famous Mermaid is there at 55' in clear water and there's a small landing craft wreck slightly deeper - maybe 70' Both are marked with balls so ask someone on shore to point them out. We found it easiest to surface swim out to the wreck, drop on it then swim back to the Mermaid.
Sunset House is also good for lunch - My Bar there is a divers bar with good sandwiches. And the Cathy Church Photo Gallery is there also - if she's in and you buy a print ask her to sign it. I should point out that the entire time you're at Sunset House you'll still be able to see your cruise ship so getting back isn't really an issue.
Sunset House Home - Sunset House
Otherwise there's Turtle Reef. IMO it's the best of the Cayman Shore Dives. It's also on the north end past all of Seven Mile Beach so would require a 20-30 min. cab ride. SunDivers is on-site there and as mentioned arrangements can be made with Happy Fish for a guide. But it's really a pretty easy dive - there's broad steps into a protected cove. A short swim out is the ball that marks the mini-reef - you'll easily spot it from shore. Drop there and proceed along the reef in either direction - it bottoms out around 45-50' one way and closer to 70' the other - farther out there's a big sand flat that you'd have to then cross and a longish swim out to the wall from there. If you head to the south along the mini-reef and you'll encounter a large group of Tarpon in a shallow cave - they've been there forever - or at least since I saw them in 2003.
For post-dive there's the Macabuca Tiki Bar on site - also you'll be diving off Cracked Conch's deck - a much nicer restaurant.
Cracked Conch Grand Cayman Ocean Front Seafood Restaurant and Tiki Bar - Turtle Reef Snorkeling and Diving Getting a cab back won't be difficult - just ask someone to call a few mins. b4 you're ready as they probably drive up from Seven Mile Beach.