you will never read the temp of the head by having cool water flow over the sender, that is defeating the object of how the engine is cooled.

get a heat gun and point it at the head when your gauge reads 50c you will see that the head temp is nothing like the water going through it.

outboard temp is always measured as head temp, not water temp...

the water is cooling the head so therefore it must be colder than the head for this to do any cooling, this colder water is also
cooling your sensor instead of letting you see the REAL temp of the head..
you are getting water temp with your set up.

I dont know if there are any specs around about the actual water temp in the cylinder head, I have never seen anything about this for an outboard !!!.

maybe someone else out there knows this spec ?