We've gone to Grand Cayman for many years (8 or 9). My uncle lived there so we went for a couple months a year. The diving is phenomonal for fish but as for coral not the greatest. The cartography is unlike anywhere else in the world and they have some super sweet wrecks to dive(USS KITTIWAKE). Although corals aren't that great, the Caribbean is not known for corals and no matter where you go the ocean is not going to look like some peoples fish tanks. It would help to know if your going to be diving or snorkelling.
If you go to Grand Cayman i suggest you also take a little trip to Little Cayman or Cayman Brac(Sister islands of Grand Cayman Cayman and much smaller). The diving is equivalent maybe even a little better. Little Cayman is known for its cool dive sites. When we went there was a site where there were 2 Tagged Nassau groupers that you could cuddle with and pet. The fish would even snuggle up to you if you didn't continually pet them Lol. (keep in mind these groupers are 3-4 feet long and are extremely endangered.
) If you go to little cayman get a guide to take you to Tarpon Lake. It is a brackish water lake that was created by a hurricane and tarpon got carried in there during a hurricane. The fishery is crazy, even if you don't fish its a cool thing to see the tarpon school chasing all the fish.
Cayman Brac is more known as a party island. I have never been there so have no personal experiences.