The 53-foot recreational boat caught fire about 4:30 p.m. while the occupants were working on the vessel, according to Chief William Laliberty. The boat was moored on the Merrimack River near Jamaica Lane. There were no reports of injuries.
Laliberty said the Massachusetts Department of Environmental Protection was notified because there was a gas leak from the boat, which carries up to 300 gallons of fuel.
Laliberty estimated that roughly 50 to 100 gallons leaked into the water but did not know for certain.
The state environmental police as well as the Haverhill Police and Fire departments are overseeing the fire investigation.