If you have an I/O drive that uses a raw water pump such as as Jabsco, either off the main crank or a belt drive on a separate housing (as I have). I was having trouble with impellers and, not knowing if the water was flowing resulting in overheating.
I switched out some of the copper fittings and black rubber hoses with clear vinyl hoses and plastic fittings that worked with the clear hoses (brass/copper are not compatible).
Now when I fire up I take a quick look in the engine bay and see the pump energize the system (via bubbles then steady stream of raw water). Then you know you are good to go.
There are no issues with temps as sea water is pretty much the same annually. It also helps in knowing when to give the system a flush as build up is evident daily.
Hope this helps. pk