I was in the keys a few weeks ago but didn't get to the lower keys except for an afternoon drive to Key West. A book I picked up for the trip shows Big Pine Key about 30 miles from Key West, Big Pine is considered one of the lower keys. Key Largo has the biggest concentration of dive sites, but there are a few sites near Big Pine, listed as follows:
***Newfound Harbor***, .5 miles offshore from Newfound Harbor Key, 15' max depth
***Looe Key (East End)***, 4.5 miles from Newfound Harbor Keys, 35' max depth
***Looe Key (West End)***, 4.5 miles from Newfound Harbor Keys, 35' max dedpth
***Looe Key (Deep)***, 4.5 miles from Newfound Harbor Keys, 40' - 80' depth
***Adolphus Busch Sr.***, 3.5 miles off Looe Key, 50' - 105' depth
***Western Sambo***, 4.5 miles south of Boca Chica Key, 15' to 45' depth.
I didn't dive any of these sites myself, stayed in the Key Largo area, but it looks like you will have some variety near by.
As far as charters, you might want to see if there are any that depart near the place you are staying. In Key Largo I could see the dive boat from our hotel room and I carried my gear by hand from the room to the boat.
Hope this helps....