I have recently been to the 3 places in discussion with non-divers. Coz is for divers and cruise shippers,IMO, so if you have non-divers I would not stay there. If you are a non-AI person like myself I would stay in Playa and/or Isla Mujeres. I would split my time because it is not difficult to get from Playa to Isla or vice versa.My diving experiences in Playa were just OK other than the dive that we saw the Bull Sharks on. Playa has nice beaches and some quaint,reasonably priced hotels and from there you can dive , easily access Coz,Tulum (ruins),Akumal(diving or snorkeling with the turtles on the main public beach) or one of the many theme parks for day trips. I would recommend spending a few days on Isla Mujeres as well. There is a safe and comfortable vibe and has nice beaches,ok diving, a couple good snorkeling sites,zip lining,etc while touring the Island on scooters or golf cars. If you wish PM me and I will provide bus ,ferry and taxi info and/or hotel ,dive,snorkel recos.
Enjoy your adventure.