My wife has some back and hip problems so we've been using the resorts that are part of Clearly Cayman. They provide a 'valet' style service so my wife does not need to walk with a tank on her back. So, we normally do a package including accommodation. We've done all three islands with them.
We've also dove a lot with Sunset Divers. They are just 20 minutes south of George Town walking.
You may also want to consider attending one of Cathy Church's annual Photofests in May. A great week of morning photography and afternoons of lectures on different topics. What I like about these is that you dive one relatively shallow dive site for the morning. As you need to, you come up, stay in the water, and the boat crew changes out your tanks. You can spend a lot of bottom time at one spot getting that one great picture you want. It's a dive week for U/W photographers.
Some of my favourite dive sites are Devil's Grotto which is reachable as a shore dive from Eden Rock divers. Also like the Doc Poulson and Aquarium. Killer Pillar has some great Pillar corals. You can see some older pictures of mine at http://caymanmarinelife.naishfamily.net/