250 is way too hot and can cause serious damage. If you have looked at your belts and they are not excessively worn, checked the coolant and it's ok, there is no water in the oil, oil in coolant or staining on the block--then this strongly suggests to me that the raw water cooling side is not working. Is the raw water seacock open, is the raw water strainer clean?
I am betting on the raw water side myself. If that stuff's ok pull the impeller on the raw water pump. It is the cause of many if not most overheats. It is a relatively simple task, requires a gasket , a new impeller and your time. You could merely inspect it--looking for cracks at the base of the fins, or excessively soft fins or even missing rubber, but the defects can be quite microscopic and still cause a problem. In MHO, replace the impeller to be sure you have eliminated the problem--they are cheap. If you find missing chunks of rubber from the impeller, try to get as many out of the heat exchanger tube as possible. They will clog the heat exchanger which will definitely cause overheating. If that doesn't solve the problem, remove the heat exchanger, take it it a radiator shop and have it pressure tested for leaks (pinholes)--those can be fixed usually--and have it "boiled" as you may have built up scale that is impeding flow and quantity. All of this stuff on the raw water side is routine maintenance which should be done periodically on all marine engines anyway and will improve the performance once you get it all back together. AND by all means, replace the zinc pencils while you have the thing apart--it's easier. If that doesn't fix it, then look into the other stuff.