My experience is with older models of Yanmar raw water
Provided your engine is also raw water cooled (with sea water) this is a solution to your problems.
Rather than blame the sensors you need to do a flush of the cooling water system.
First of all, check the annodes in the system. Are they corroded away? Have they expanded with crumbly dross? Remove the annodes from their housings (bungs) and reinsert the bungs.
Next remove the thermostat and cover.
Next remove and check the water injection point into the exhaust elbow
is clear of dross and blockages.
Also check the condition and replace the raw water impellor if necessary. Chances are it has been working hard.
Drain the cooling salt water
in the engine.
Mix a solution of gellcoat restorer or rust converter (phosphoric acid) with fresh water, try a weeker solution at first. Pour it in to the thermostat hole. This should begin to froth up as it disolves the calcium deposits around the cooling galleries. Flush this out by removing the cooling water exhaust
injection hose and attaching a fresh water hose. Do not use heeps of mains pressure or volume, just allow the hose to run. Remove the annode bungs again which should allow the dross to escape as well.
Now depending on how much crap comes out you should repeat the process a number of times until you are satisfied the galleries are clear. You may like to increase the concentration of acid but be warey.
Once satisfied you have removed the dross from the system, install new annodes, replace the hoses, thermostat and cover.
Run engine and monitor
guages. You should see the engine restored to its normal operating temp.
The best way to prevent this happening again is to allow the engine to idle for a period of time to equalise the thermal tensions within the motor. I reccomend installing a fresh water flushing
system which replaces the salt water
while iddling down.
Add a hose from your water tank via a tap and T piece to the cooling water inlet between sea cock and engine raw water pump
. Open the fresh, close the salt
then time how long it takes before fresh water comes out at the exhaust. Friends of mine used the sink drain and a sink full of fresh water to insert fresh water into their yanmar after each run.