Good point. FWIW, I pass along this suggestion from Rob at Depco Pump. Last year he mentioned to me that it's a good thing to do a freshwater flush, ideally with dish soap in it, particularly if the boat is going to sit for a while. He said that this is good not only for the impeller but also for the pump seals.
In my case, my Yanmar 1GM is a raw-water cooled engine, and so I have fitted it with a Y-valve so I can draw in fresh water from a bucket to rinse it out after (ideally) each use. This is to help keep the engine internals free of salt, but it also takes care of the pump/impeller issue at the same time. Perhaps those of you with FW cooled engines might want to set up something like this for at least occasional flushing of the pump and impeller.
Note that my boat is in salt water, so maybe this is not as crucial on a freshwater lake.