One easy way to have an alarm, especially on smaller diesels like many sailboats have, is to clamp a "snap disc" thermostat on the
exhaust elbow. Just wire it to 12V and to an alarm buzzer. They are available in different break temperatures. I've used 185-200 F in the past. If water stops flowing to your
exhaust, your metal mixing
elbow gets hot, the snap disc shuts grounding the wire, this sets off your alarm buzzer. Very inexensive. Available at Graingers and probably Mcmaster Carr. I think they are available in adjustable ones also.
For the bigger
diesel crowd, good idea to have exhaust temp gauges. When a
diesel engine manufacturer certifies an engine
installation, they want to evaluate exhaust temperature.