I don't have good flow and that was why my thinking was the thermostat. It was ok at first at slow throttle and like you said it started overheating just above 2000 rpm and then it was all she wrote as she stayed hot.
Umm, the thermostat is on the fresh
water side of the cooling system. If you have low raw water flow, your problem is on the raw
If you've checked the strainer and have good flow from the seacock, then it's probably either the raw water pump, the heat exchanger or the exhaust elbow. The impeller may be failed but not look it - take off the cover and turn the pump with the belt by hand - make sure the impeller itself is in fact rotating. Then check the heat exchanger and the exhaust elbow.