In addition to controling water pressure, the poppet valve also bleeds air from the cooling system. Look at the diagram Bigshrimpin posted. The hose fitting that feeds the poppet valve is at the top of the block where air pockets naturally form when the lower unit takes a gulp of air. Purging air from the system is just as important as controling pressure and flow. Air pockets in a water cooled engine = hot spots. Hot spots create stress when a part of the engine undergoes thermal expansion at a rate different from the part next to it.
Likewise, the T-stats are important for keeping the engine at the correct operating temperature. The rapid heating and cooling that occurs when loading and unloading an engine with no T-stats, and therefore no temperature control, create stress due to repeated, and often uneven, thermal expansion and contraction. That can't be good.
I agree with all of Fr. Frank's recommendations, but I've never run dilute muriatic acid through my motors. The slightly acidic solution I run mine in is the Suwannee River.
Dave