To stabilize combustion in an engine and to sufficiently cool an exhaust pipe.

A cooling water path in which cooling water supplied from a jet propeller 27 flows has a cooling water path for engine A cooling a cylinder block 21a and a cylinder head of an engine and a cooling water path for exhaust pipe B cooling an exhaust pipe. Since this cooling water path for exhaust pipe B is connected with the cooling water path for engine A on the upstream side of a positive pressure part of the jet propeller 27 or a thermostat 65, cooling water quantity in the cooling water path for exhaust pipe B fluctuates little, even when the cooling water path for engine A is opened and closed by the thermostat 65 and a water temperature of the cooling water path for engine A is controlled constant.