How did you check the water flow to the tstat? Just curious.
Cold start up and your temp is 180? That’s not right. What did it used to do. It should climb up slowly to 160 or so. You could be sucking air somewhere, this will not show on a hose as the intake is under pressure. Blipping the throttle in neutral should always bring the temp down immediately to its NOT. Maybe your impeller housing seal is shot.do you have a water strainer? Did you try opening your exhaust drain to verify water is being supplied to the motor while on the lake at idle?