Try WalltoWall or Neptune's both take 8 max only. Neptune's has the bigger boat. Just about every dive op will pick you up so
nearby isn't really an issue unless you have a reason for it. Given what you're looking for, I probably wouldn't dive with Red Sail, the diveop at the Westin. Their smallest boat takes 12 and their larger one(s) up to 24.
Why? - because they post here and nothing I've read disagrees with my experiences there. The diveop I dove with didn't survive hurricane Ivan or I'd recommend them.
http://www.walltowalldiving.com/index.htm - PM CJ's here.
http://www.neptunesdivers.com/index.html - PM Caymancase.
Aquarium is nice, Hepps Wall is nice, Peter's Reef is nice. I didn't exceed 50' at any of them. Although you can drop over the edge and go deeper at Hepps. The trend in Cayman is to go deep first off/near the wall then do a shallow second dive. In some cases it's possible to stay shallower on top of the wall. So mention your experience level to anyone you book with.
If you feel like shorediving, drive a couple miles north to Divetech's Turtle Reef location, there's a shallow mini-wall there that I think is the best shoredive on Cayman - easy access into a protective cove with a short swim out to the site also.
And Stingray City's been called the best dive you can have in 15' of water. Most of the boats that go there leave from the Harbour behind your resort. Book it through whatever diveop you select.
The vis should be great, typically 90-100' unless a hurricane churns it up. It's partly why Cayman is one of the top Caribbean destinations.