Steve,
I am glad to hear that it paid off in more than just "peace of mind", but sorry you actually had an "issue".
I highly recommend them for everyone. They are not expensive (actually cheap boat expense wise) and are fairly easy to install. They will instantly notify you with a very loud, distinctive alarm for any of the following: forgot to open thru hull, some thing blocked the thru hull or clogged the strainer, failed impeller or pump, major hose leak before the alarm, or even a restriction in flow anywhere in the raw water system. You would think I get a kick back or something, but no just a happy customer.
The other alarm I swear by, and the one that like you just described for your Aqualarm, saved my bacon was the Borel Exhaust hose alarm (Aqualarm makes one as well, but the Borel alarms at a lower temperature). It is also very inexpensive and easy to install and can be monitored by the Aqualarm monitoring panel for 2 alarms. The first place that will overheat, if one is starting, is the exhaust hose. This will set off the alarm long before any high temperatures occur anywhere in the engine that could cause major damage well before the coolant alarm goes off. In my case this Borel alarm went off because my exhaust elbow had corroded through on the inside causing a poor pattern of cooling water inside the exhaust hose resulting in several hot spots. Without this alarm, I would not have been able to tell I had a developing problem as the rest of the entire engine and cooling systems were well within normal operating temperatures. However, failure to act on my part could have resulted in several bad issues. The exhaust hose could have overheated and failed sending hot saltwater into my ER, bilge, and on the engine, as well allowing CO to enter the boat!
Prior to that, it could have allowed salt water to enter my turbo or even allowed it to enter the engine exhaust manifold and valving. This saltwater issue could have gone on undetected for a long time, ruining my turbo and/or engine. This could have been very expensive, and potentially dangerous!! Also, if you have a hose failure, past the Aqualarm, that alarm would not notice a problem because water would still flow, however the Borel would notify you very quickly as the exhaust hose would get hot fairly quickly and that alarm would notify you again long before a true engine overheat occurred.
As you can tell, I am a proponent of both types of alarms. Approximately $200 total for both (parts only). Many here spend that on a nice dinner out with your partner. Cheap insurance in my book.