The boat has a Yanmar diesel. It's a 2QM15. That means its two cylinders and 15 HP. The marina guy said the motor ran but it needed a water pump. Sure enough I got the motor started and within a few minutes it started over heating. You know that because bells go off on the control panel. I started trouble shooting the motor. At that point I knew nothing about diesel motors. I now have a PHD in diesel.
The first thing I did was take the water pump off and check the impeller. It was fine but I replaced it anyway. Started it up and still the same problem. I got the diagram of the motor online along with the entire shop manual. Great help by the way...
The next thing to check was the zincs. Zincs are things that protect the motor from electrolysis and damage from that. They get eaten away first so the motor don't get sacrificed to the electric gods. I pulled both zincs out and what was left of them disingrated in my hands. From that I also knew that there were particles left inside the motor from the zincs. I cleaned out as much as I could and replaced with new zincs. Looking to a good running motor. No such luck. Still overheated. The next line was to take the exhaust manifold off and check for blockage there. I started removing the part, starting at the back end where the water exits. On that part was what was called a bypass tube. I pulled that off and it was completely blocked with crud and scale. I cleaned that tube and reinstalled everything. The motor ran like a top.
I changed all the filters and the oil so we were good to go. (except for the muffler!)