no marina in Key Biscayne, excpet the KBYC which offers reciprocial privileges to other YC
Crandon MArina is between miami and KB, but you'd be stuck there, must have a car
Dinner Key in coconut grove, Miami Beach or Sealine (the old mariott... whatever it's called these days) are your best bet.
if you jsut want to enjoy the beach on Key biscayne, then you can just anchor in No Name Harbor, by Cape Florida, it's a state park. no power, or anything but it's a real nice park and within walking distance of the village (shops, restaurants, etc...) nice stop over, good beach, nice trails, good holding.