Sky 10 was above the marina at 4000 Crandon Blvd. just before 8 a.m. as firefighters worked to extinguish the flames from the dock.
A fireboat was also nearby.
Several boats were berthed near the affected boat.
Miami Fire Rescue Capt. Ignatius Carroll said three boats were destroyed and three sustained heat or smoke damage.
According to Carroll, some of the damaged boats were berthed next to each other, while others were in different areas of the marina.
Carroll said investigators believe the fire spread to other vessels by a small boat that was burning as it floated down the water.
The cause of the fire is under investigation.