From US Coast Guard:
§ 182.425 Engine exhaust cooling.
(a) Except as otherwise provided in
this paragraph, all engine exhaust
pipes must be water cooled.
(1) Water for cooling the exhaust pipe
must be obtained from the engine cooling
water system or a separate engine
driven pump.
(2) Water for cooling the exhaust
pipe, other than a vertical exhaust,
must be injected into the exhaust system
as near to the engine manifold as
practicable. The water must pass
through the entire length of the exhaust
pipe.
Now ya know.
Dan'l