No injuries were reported after the fire erupted about 4 a.m. at the Pelican Cay RV Park & Marina.
A 50-foot boat was fully engulfed in flames when firefighters arrived, Monroe County Sheriff's Office deputies said. A 25-foot boat also caught fire.
The larger boat broke free of its moorings at one point during the fire.
Campers were forced to evacuate during the fire.
The state fire marshal is investigating the cause.