Re: alarm buzzer came on when starting the motor at the ramp
Well, you don't have to worry about a low oil level causing the alarm because they used premix back in those days and there is no oil container to remember to keep filled.
So, you are after a cooling solution.
Like you said, if it went off immediately, you either have a defective switch or a short in your wiring.
If I were in that situation, I'd remove the cowl from the engine and feel (and keep an eye on) the cylinders and heads water jackets. If that engine is getting proper coolant obviously they will only get warm over time; no coolant and it's ouch to the touch and that can come in 5 minutes or so.
Thermostats start opening at roughly 140F The water heater in your home Normal temp setting is designed to deliver 140F water to your faucet for a benchmark as to how hot is 140F. Overtemp sensors are around 190F roughly. Water boils at 212F. Engine parts should be good to over 250F to give you some more ideas as to what's ok and what's not.
I'd go fishing and not worry about it (making the checks mentioned) till I got back home. If you are worried about getting out and not back, if the problem really is circulation, run till it gets hot to the touch, shut it off till it cools and repeat the process till you get home.
My 2c
Mark