I have a Nissan 3.5 and have motored a small boat for 2 seasons and used it for an outboard on my C22 this season. I have only ever run the motor once in a freshwater tub and it''s still running fine and squirts out warm water.
I will tend to agree, flushing after each outing is not required. However, it can''t hurt the motor either.
A seasonal freshwater run will be what I use from now on.
Tim