I too like the Borel exhaust temp sensors and alarm module.
High exhaust temp covers 99% of the potential raw water related overheating issues and I can't think of the other 1% but I threw it in just to be safe. It is entirely electronic, so it does not depend on a mechanical flow sensor like the Aqualarm that can foul and fail open.
The Borel alarm system is a simple signal to ground to alarm, so it can be used with any other sensor in parallel with Borel's exhaust hose band. For Cummins 6BTAs which have a fluky stock coolant water overheat sensor location, it can be used in conjunction with another coolant sensor mounted in a place that will be quicker to respond than the stock one.
I believe that Ski uses something similar, although home brewed I think on his Cummins.