Melandre wrote:
No, it is (unfortunately) just a standard cooling system (it is only
17.5' boat). Simply put, recently the boat starts hot (needle creeps
to the 200F to 250F range) and after a random period of time (averaging
1 to 4 minutes) needle suddenly drops to around 150F and all is well
and engine appears to stay at approx. 150F.

If it was the impeller or clogged hoses, wouldn't the problem continues
after 3-4 minutes of running? Andre

If you are dropping back to 150 degrees after an initial heat up to
200-250 degrees, it does indeed sound like the thermostat. Impellers,
hoses, obstructions, etc are permanent situations, not self correcting.

If you test the thermostat in a pan of water, (using a candy
thermometer to note the temperature at which it is opening), and it
appears to be opening within specs you might need to go to a thermostat
with a lower rating. There's no good reason to heat up to 250 degrees
if your engine is happily running at 150.