Now that you own a boat with the same Northern Lights generator as me, I thought you would be interested in a repair that I had to make that really surprised me.
For several months now--including last fall--my generator would work for a while and then quit because of overheating.  The exhaust had water coming out but at a reduced flow.  So, I checked the inlet filter--perfect, the water flow without the filter in place--lots, the pump impeller--perfect, the trap before the heat exchanger--clean except for a small number of broken parts, the heat exchanger tubes for objects--clean, and the coolant level--fine.  Everything looked fine.  During the course of my eight years of ownership I have had to replace several impellers and never found many broken off pieces.  I always assumed that they must have been small enough to go through the heat exchanger tubes and out the exhaust into the sea. 
So, once again, I do the whole thing over again, and again have no results.  So I decide to check the input hose where it attaches to the impeller in case some vegetation got by the filter--which I doubt as the filter screen is perfect. 
Surprise, surprise!  where the hose fits over the pump intake nipple are all the missing broken off impeller pieces--see attached picture.  Remember, this is the input side of the pump, not the output side.  The pieces completely filled the tube to nearly half an inch.  I was surprised that any cooling water got by this blockage. 
I now have lots of water coming out of the exhaust, and the generator can handle every load that I put on it.
If you find this interesting, you may want to put it on our club website.
May the wind always be on your beam.