a frame of a water jet propulsion unit arranged to be installed and sealed into an installation opening of a boat . the water - jet propulsion unit includes an impeller and the boat has a counter - sealing surface to which the frame is attached . the frame includes a flow channel to guide water to the impeller . the flow channel has a top with a maintenance opening . the maintenance opening is positioned to permit servicing the impeller . a first sealing surface forms a first closed loop to attach the water - jet propulsion unit tightly to the counter - sealing surface of the boat . a second sealing surface forms a second closed loop arranged to attach the water - jet propulsion unit to the boat . the first and second closed loops permit the maintenance opening of the frame to be alternatively locatable respectively outside or inside the stern of the boat in dependence on alternative configurations of the installation opening of the boat .