There are a couple of different ways
There are a couple of things you can do. Disconnect the exhaust hose, or water line at the mixing elbow, and using a bucket, check how much water is actually pumping through the engine. A Yanmar manual will tell you how much water it should pump. Second is to install a temp guage, which will eliminate any doubt about engine overheating. If you buzzer and sending unit are working properly, you will get the warning buzzer if it's overheating. You should make sure the sending unit is for a raw water cooled engine. Fresh water cooled one is much cooler than the other. There are also other things that will cause it to run too hot. Mixing elbow, which if it has never been changed on that year model motor, should at least be checked. The raw water strainer can easily get stopped up with weeds or crud. Thermostat that is bad, and not opening properly. One thing I strongly recommend, is find out for sure that if is in fact running too hot, and the exact cause, before you start buying parts and pieces. Replacing parts before being sure of the exact problem, can get very expensive, and frustrating.
Dream like you will live forever. Live like you will die tomorrow. ( James Dean )