Regarding specific topics in your post, a raw water (or sea water) pump impeller is first thing to try. The raw water pump draws water from the lake/ocean into the cooling system. The impeller is a 2 year replacement item and also the first thing to check/replace in an overheat situation. The raw water pump is belt or crank driven on the front of the engine. Usually the impeller is easily replaced by removing the front plate of the pump. Just replace, it is necessary to remove it anyway to inspect. If pieces are missing then the fun begins in tracking them down and flushing out.
If it overheats only on plane then there are some plumbing type items to look at and replace in the outdrive, as the water is drawn up thru there and if any components leak aire then suction is broken and there will not be sufficient water supply to the cooling system. At idle speeds all that is underwater so any air leak cannot not show up then.
Next is heat exchanger, like radiator in the car, only liquid to liquid not liquid to air. It has two chambers, One flows the water from the raw water pump thru, and cools the engine coolant in the other chamber. The engine coolant side is closed and has a radiator cap. Either of those chambers can be plugged or leaking. On the raw water chamber it has a strainer also that needs to be cleaned out regularly. That chamber can also be where a piece of a broken impeller can lodge impeding water flow.
There is more, but depends on some sleuthing and actually running it to see all symptoms.
