I am planning a wedding for about 70 on the beach so I wanted to pick a place that had beachfront property, so no need to shuttle that many people around. There are very, very few places with their own beaches, but with 20people you could easily shuttle over to Smather's Beach and then off to a reception elsewhere.
I contacted a few planners in the beginning and liked the
Key West Travel - Group Travel Planning - corporate incentive travel and events planner for Key West Florida company the best. I'm not using a planner as the resort we are at supplies one. They have so many neat ideas. You can do a catamaran cruise the night before or check out the Audobon Gardens for the ceremony if you decide against the beach.
Also remember that hurricane season lasts for 6 months out of the year!