Crews were called to the Brookfield Bay Marina shortly before 3 a.m. after getting a report of a boat on fire.
When firefighters arrived, they said they found a fully involved boat fire with exposures to surrounding vessels.
Multiple fire crews extinguished the fire from both sides of the cove, firefighters said.
Crews from New Fairfield responded for mutual aid and crews from Danbury were contacted for additional Hazmat resources, officials said.
The state Department of Energy and Environmental Protection was also called for the Hazmat incident on the lake.
The cause of the fire is under investigation.