A pipe of too small a bore will restrict the flow of water to the engine.
At least part of the pipe between the inlet valve fitted to the hull and the pump mounted on the engine must be flexible to allow for engine vibration. Use two hose clips for security at each end of the pipe, and make it long enough to allow plenty of movement of the engine without imposing strains on the pipe and inlet valve.
Special water hose is manufactured from a different material from fuel hoses and will not be chemically affected by the water. Garden hose-type piping should not be used as it has a tendency to kink, reducing the flow area and causing inadequate water flow and overheating: genuine water hose is reinforced with a spiral steel inlay to prevent this. The hose should be insulated and fire resistant to a point above the waterline.