[tsubaki]

Haven't had carbon build up on outboards but in auto's the basic principal is scoot water into the intake while running at moderate rpm's. This will create a white emission out the exhaust and cloud the neighborhood.
Done incorrectly it will warp or bend valves. The premis for store bought decarbon additives is much safer. The additive is petroleum based and the oils attach to the carbon and fall off. This process is identical to fuel injector cleaners.