We've stayed at GC Marriott 5 times and once at Casa Manga. Seven Mile beach in GC has all of the things you're looking for. Fantastic snorkeling from the hotel's beach. The stingray excursion is a must do. Plenty of restaurants are within walking distance of the Marriott. Be aware, as VickiSoCal mentioned, the pool is small. But who needs a pool when you have a world class beach steps away. We also like that the airport is 10 minutes away by taxi ($20) and the basic standards of the island are comparable to the US. You can drink the water, sanitary food prep, relatively low crime rate, sidewalks, street lights, etc.. The only downside (in my opinion) is that everything tends to cost more in Cayman. Expect 20-30% premium over average US prices.

Casa Manga is more of a mega-resort. Huge hotel. Huge pool. Nice swim up bar. Decent beach. However, when we were there (end of May). The surf was too rough to swim so we spent most of our time in the pool. Otherwise, we found the hotel to be nice and very affordable. Decent prices of food and drink. We didn't leave the hotel on this trip. So I can't advise on the area.

I hope this is helpful.