The most likely problem is the water
impeller. These take a "set" over time and don't pump as well or at all. Sometimes by running for a few minutes until it begins to overheat, stop, let it cool and try again they will start pumping again.
But in most cases, just replace it. On my Evinrude
it was a bear. Easy to disassemble, just a couple of screws on the propeller
housing and gears. But to reassemble you had to align three tubes simultaneously: gear
shift rod, drive shaft and raw water
tube. All had to align perfectly and it took me at least an hour of fiddling to get it all in place.
This is one chore I would gladly leave to the pros, at least on this Evinrude
. Hopefully yours is easier.