PURPOSE: To enable a screen to sufficiently withstand water pressure in sucking of water into a suction port by action of a water injection unit, when the screen for covering the suction port formed on a bottom plate of a hull, and also to enable propulsion performance of the water injection unit to be maintained well.

CONSTITUTION: Water 30 sucked into a water jet duct 18 of a water injection unit is injected backward from a hall 3 through a suction port 19 of a bottom plate 5 of the hall 3, so as to advance this hull 3. A screen 49 for covering the suction port from the lower part of the hull 3 is provided. This screen 49 is composed of a plurality of longitudinally extended rod-like bodies 52, a front plate 50 for connecting the front ends of these rod-like bodies 52 to one another, and a rear plate 51 for connecting the rear ends of the rod-like bodies 52 to one another. The right and left both sides of the screen 49 are clamped to the hull 3 at the right and left sides near the rear ends of the rod- like bodies 52.