If you have reasonably easy access to the front of the motor an easy check for cooling water flow is to check the temperature of the front cover of the impellor housing with the back of your fingers. If there is limited or no flow this plate will heat up very quickly after the motor is started, It should always remain "cool" to the touch as the motor comes up to temp.

On a stern leg is there any visual check that water is coming out when in the marina? Seems hard to tell with the exhaust bubbling away.

The temp gauge went up a long way but I'm not sure where it stopped as I was too busy concentrating on slowing down, turning it off, and then checking the engine - and cursing a bit.

sighting exhaust water flow before moving off should become a subconscious habit in time

as suggested above - a check list for getting started and closing up at the end of each trip might be something to develop. i have been considering one for myself

Personally I don't close the seacocks unless I was leaving the boat unattended for a long time. If you inspect them regularly they are a low risk item.

Sounds funny that you got hot enough to be dumping coolant without an alarm? If it is just set too high you can possibly get another one with a lower set point and just swap it out. Did you note what temp was on your gauge?

Is it normally okay to open the seacock and start using the engine again if it's staying cool, or is it likely the impeller or something else is already damaged?

Q1: Do most boaties leave the raw water seacock open or closed when berthed?

Q2: Do you inspect that it is open before each trip? or only after someone else may have been on the boat?

From what I experienced I still think the temp gauge and alarm is too late in this case in alerting me. Something should have alerted me when I idling slowly out of the marina, which is why I started this thread to begin with.

Would of the exhaust temp shown something was not right earlier?

I hate having a crash course in learning thse things....