Crews responded to the boat ramp at 11:43 a.m. to find a 25-foot cabin cruiser engulfed while still on a trailer attached to a truck.
The department said it battled the fire for about half an hour.
Authorities estimate the fire caused $20,000 of damage to the vessel and about $5,000 of damage to the ramp.
The department believes a backfire combined with gasoline fumes in the engine compartment may be the cause of the fire.