I got curious regarding the discussion of rinsing an outboard using the
garden hose flush port versus muffs on the water inlet so I did some
reading. I had a Yamaha 250 four stroke on one of my outboard boats
and seemed to remember reading something about it.

First of all, it is not recommended that the engine be run while
flushing using the flush port. Yamaha states that *if* your water
supply pressure is sufficient to generate a steady stream from the pee
hole, it's ok to run the engine while flushing at idle RPM and for no
longer than 15 minutes. If the flow is *not* sufficient to generate a
steady pee stream, Yamaha says to use muffs in addition to the flush
port if you want to run the engine.

The flush port injects water after the thermostat which will be closed
if the engine is not running. If somehow water can still get down to
the inlet passages and impeller, the impeller won't be turning (unless
the engine is running, in which case it would simply pump the water back
in the opposite direction), so I don't see how it can be rinsed off.
The impeller would tend to seal off any water from running through it
with the engine not running.

So, I still question if using the flush port thing really does anything
to rinse the water inlet and impeller, extending it's life.

Here's some collaborating information:

http://www.saltawayproducts.com/FlushPortPage.htm