A wooden ferry caught fire in Nam Sang Wai in Yuen Long on Monday morning, with authorities saying it was suspicious and classifying the case as arson.
The sampan was used to row people across a river to the village.
It's the third blaze in the important bird nesting site in two months. The fire, which began shortly before 6.30am, was put out in about 10 minutes. No one was injured.
Last week, the government rejected a call to take control of Nam Sang Wai in Yuen Long, after two fires were reported near the popular bird watching site.
The fires, one of which is suspected to have been an arson attack, left hectares of land scorched.
But the Environment Secretary, Wong Kam-sing, told lawmakers that the damage was not too bad, based on the initial assessment by the Agriculture, Fisheries and Conservation Department.