When I go to Key Largo, I stay at the Sheraton beach Club resort mile marker 97. Clean, not too expesnive (less than the marriott by about a third), nice beach (not common in key largo) with water sports available on site, with good dining. Nice pool, rooms are a good size. Beach front room about 120 or so a night for two queen beds.
I dive out of Ocean Divers, MM 100. They do two tank am and pm dives, about 65$ per person for two dives I think. Boats are nice, although they can get crowded in summer. I was just there last weekend, had about nine to ten people on each trip.
Weather and viz were perfect, but I've been there in five to six foot seas and half the boat sick, too. Keep an eye on the Molasses reef buoy data on the national Weather service website. It gives the local wind speed and direction, air temp and water temp. The days when the wind is less than 10 kts and from the north or east, great. If the wind is 15 kts or more and from the south, it's fish feeding time. The water temp will give you an idea of the exposure protection needed.
Look up an instructor named larry gates. He is a professional guide, very nice, very reasonably priced who knows the reefs like the back of his hand. he might help with the younger divers... he also takes great photos of your dive... check out his website larrygates.com