Water Pump - Impeller

So, taking apart, piece by piece, Athena's engine's saltwater cooling system.
Hoping to find the "smoking gun" causing the engine to overheat.
Typical first thing to check when engines start overheating is the water pump or impeller.
The impeller is not an easy thing to get to...
Top left pic is a view of the water pump looking forward from engine room (note for size, the circular cover is only 2.5 inches in diameter, and although those little bolts may look deceivingly accessible in the photo, the bottom three are actually impossible to get to). Top pic on right is a view of the water pump looking aft from the saloon (the water pump is behind and attached to, pumped-by, the black drive wheel beneath the large black hose). The water pump and drive wheel had to be removed in order to remove the circular cover of the water pump to access the impeller. It's also nice to have the pump removed for easy access for removal of stripped bolts... mini vice grips are great for removing stripped bolts. this situation will hopefully be remedied with different type of new bolt (more to come on that later).
Once opened... (bottom left pic is impeller in chamber), low and behold, there was some hard fleck pieces within the impeller chamber... (bottom pic on right shows flecks) but, enough to be the smoking gun....????