To remedy this problem at minimal cost, I bought an inexpensive indoor-outdoor battery operated thermometer at a discount store. This thermometer has a remote sensing device on a long wire that I snaked through the cockpit locker next to my Yanmar diesel engine.

I attached the sensor to a hot water pipe on the engine with a bit of flexible copper wire and attached the read-out unit to the locker bulkhead with velcro.

While this means of taking the temperature of the water in the engine will not give you an accurate reading of the exact temperature, it will give you enough information on how your engine is running to know if all is well or if you are on your way toward an overheat situation.