Re: 78 Evinrude 115 thermostat
The way the overheat sensors work is that they ground out to the block when they get too hot. The buzzer is powered off the ignition circuit in the remote and when ground contact occurs in the sensor it goes off. If the leads from the sensors are shorting anywhere it'll trip the buzzer as well.
Next time the buzzer goes off, just disconnect the sensors (they're in the head and have a brown wire) at the connector near the heads. I think you have one on each side, but some models only had a sensor on one side. If the buzzer still sounds after disconnecting the sensor, then it's either the brown wire or your buzzer that's malfunctioning.
If your buzzer is sounding, and you're certain the block isn't getting too hot, then replacing the thermostats will not fix your problem.