Zebra. Mussels!!!  
For those whose boats are on the hard over winter, especially in fresh water (Great Lakes, Upper Mississippi River). Before you launch your boat, check EVERY water intake or discharge orifice, and speed paddle wheel (starboard side, about 39”, one meter forward of mast) and dig out zebra mussels or other growth that blocks water flow.  Might be the cause of motor over heating this summer.  
- also check the gaps at the rudder and the gap where propeller shaft enters the hull.