Here’s something to think about while manning your helm on a long dark seemingly endless night. What happens if you suck an obfuscating plastic bag or some other chunk of debris into the intake that supplies a given engine with cooling water? The answers are numerous, of course, but let’s focus on just one.
The temperature of said engine’s exhaust will rise much faster than the temperature of the engine itself. And while it’s undeniably dangerous (and potentially catastrophic) to operate an engine on little or no cooling water, there’s at least one bit of good news that adheres to this little factoid and here it is. Let’s say, for the sake of brief explication, that you could measure the temperature inside an engine’s exhaust elbow, hose, muffler, or some other exhaust-related component. Could you not then interface this capability with an alarm (either visual, auditory, or perhaps both) at the helm of the boat and thereby virtually nix the chances of burning up an exceedingly expensive diesel or gasoline powerplant by failing to diagnose a raw-water cooling problem in a timely manner?
Heck yes, you say! And indeed some boatbuilders these days are already doing precisely this, one way or another. But just in case you’re not one of the lucky few with an exhaust alarm system onboard, there are several after-market systems that can be easily added to your boat’s array of auxiliary equipment and which will produce pretty much the same result as factory issue. Installations tend to be fairly straightforward. Typically, you merely plug a sensor into a given engine’s exhaust hose or elbow, do the same with a monitor of some description at the steering station, deal with a few electrics, and off you go.
Vetus makes a product of this type, by the way. They offer sensors that mount both to exhaust hose and to an engine's muffler. See the sensors and alarms here.