After 30 years of successfull industrial trouble shooting here is my Order of Battle....
1) Check the alarm sensor
Buy a $6 digital cooking thermometer or laser pointer. Run your engine till alarm. Listen for the water burping in exhaust. Shut down engine. Quickly touch or laser the anti-freeze tank. Open the fill cap slowly to prevent hot sprayback, stick in thermometer and if it is about 160-180F
Then... it is the Sensor/alarm.
If cool anti-freeze/fresh water, then go to step 2.
2) Check the ocean water system flow
If you heard no spitting/burping in your exhaust in step 1, then it means little or no sea water flow.
Pump impeller replacement is not JUST impeller to the standard rubber flex vane pump. You have a wear plate and a paper thin gasket, if not right, that pump will "by pass". If the old pump impeller had missing rubber... ouch.
Clean your pump stainer. BTW the '
4" water' means that the top of that hose is at least 4" below the Actual Water Line. If it was say 12" below AWL you have a restriction from that hose back to hull. In other words you can measure pressure drop (restrictions) by inches of water spout relative to AWL.
3) Sea Water/Anti-freeze heat exchanger
If sea water flow is good then this should not be the problem...
If not, that has to be pulled/cleaned of fouling from crud or rubber impeller parts (see above comments and tips).
Why did I do 2 & 3 before the next part 4+? It is a toss up, but i think routine required maintenance is easier to check and clean before engine component work.
4) Replace your thermostat
I would do it new versus test. They are very cheap and you get renewed.
5) Get a Mechanic
Sorry but it highly unlikely that the anti-freeze coolant system beyond a full liquid/theromstat is the problem. and the rest "aint routine"
:cry:
We have all been there probably with vehicles or boats...
Good luck
Jim..