I’m with lars on this one, the closest United gets to the Key’s is Miami. Fly into Ft. Lauderdale, maybe stay with JupiterMermaid and dive with Jupiter Dive Center, she’s a DM there or Jim Abernathy’s scuba adventures in WPB and when I’m in SE Fl. I always do Blue Heron Bridge at least once.
If you do go to Key Largo Ocean Divers would be my first choice too, a little farther down a1a is Conch Republic.