Do you have waterflow through the engine or all water goes through the bypass line?
Rubber hose in front of the engine, from front zinc housing to collector is a bypass line. Plug it somehow, remove the thermostat and try to run the engine. Most likely you will get no or very little water. If so, your engine is blocked by buildup. If water is running freely, you need a new thermostat.
s/v NEMO - Freedom 28 Cat Ketch, centerboard