Hi Gussy,
Yes, unfortunately, no reef snorkeling offshore. I always recommend that my customers head to Annes Beach in Islamorada for a spot of beach sunbathing and swimming. Chesapeake is nice, I have stayed there before, bit quiet maybe.
In Key Largo (better diving than Islamorada) the Pelican is nice. Loads of similar resorts with small artificial beaches on the bay. Make sure your non-divers have a rental car and an idea of where they are going to visit (Theatre of the sea, Diving museum, Feeding the Tarpon at Robbies, Miami South Beach, or whatever.) There arent many resorts in the keys where you can entertain non-divers for a significant amount of time on property without them driving around a bit. nearest you could get would be the Key Largo Grande or Holiday Isle (If they like that sort of young partying holiday camp type thing)