With open cooling the cooling system has no pressure. Bubbles form in water on surfaces inside the motor between 170 and 180ƒ (depending on altitude). Those bubbles prevent efficient transfer of heat from the metal to the water. Best temperatures with open cooling is under 170, I run mine at 150 since everything I run at is over 2000 ft ASL..
Safe operating temperature is higher as the motor is designed to run over 200 with a pressurized cooling system. As well, antifreeze raises the boiling temperature of water. And the fuel burns more efficient up there as well. Another reason to own closed cooling :^) But once you get up over 190ƒ the plain water in the cooling system will be boiling in places, causing bubbles and steam and preventing efficient cooling.
I don't have an alarm on my older boat. I would hope to be paying attention and see if idling, or running at a little higher rpm would resolve the overheat. For me, if I see over 170 I am going to sit up and take notice, but I'm not going to shutdown until I take a couple minutes to figure it out. If possible. On the water, possible reasons to overheat are many, most common is picking up something like a plastic bag that covers the intake ports.
