This is a good discussion.  I have had three forays into this arena:
1.  I once manually set my Dessalator fresh water flush valve to the flush position, got distracted and pumped 800 liters of "flush" water thru my membranes.  This while motoring into Providencialis in the Turks and Caicos (the lesser draft may have aided in this circumstance).  Only when I investigated why my water tank was empty, did I discover my forgetfulness induced error. In this instance my fresh water was pumped overboard while coming into port, so no major issues.

2.  Once while under sail and running the water maker, I was surprised to hear the bilge water-level alarm go off,  I raised the engine compartment cover to see water sloshing about just around the feet of the Calpeda  AC seawater pump.  A quick finger lick proved it to be salt water and investigation revealed it was coming from the seal of the low pressure Dessalator feed pump.  I assume my bilge pump had been operating for some time until it was overwhelmed.

3.  While sailing from Grenada to Carriacou one boisterous day I overheard a one sided MayDay discussion and noted we were only about 5 miles away from the lat-long of a vessel that was sinking.  We arrived first on scene about 20 minutes after the 4 folks took to their life-raft and watched their home disappear into 11,000 ft of water 2 days before Christmas.   Getting them and their life-raft aboard in 25+ knots and 8 ft seas was quite an adventure. They were sailing along, when the wife decided to go below and get something to eat only to find the floor boards floating.  All attempts to find the site of the water intrusion were futile as the water was already too deep.  Early warning may have helped them save their boat. 

This got me to remembering my first offshore sail training aboard the Orange Coast College vessel Alaska Eagle en-route from Costa Rica to San Diego via Clipperton Atoll.  When standing watch we manually pumped the bilge every hour and logged it.  Able then to deal with any increase in bilge level.  At any rate, after one or more of the above episodes, I purchased a bilge pump run-on alarm from Borel Manufacturing of California.  Wired into the bilge pump circuit, if that circuit runs for more than 2 minutes it sounds a 110 dB sonalert that will awaken the dead.  Once the pump stops momentarily it resets the solid state timer of the run-on alarm.  When aboard I test it daily by switching the bilge pump to the manual position for more than 2 minutes.  

A similar alarm could be utilized on the fresh water pressure pump.  It would only activate if the pressure pump (Amfa or otherwise) ran for more than 2 minutes.  

I checked the Borel Mfg. web site just now and don't see this alarm offered currently, however, I have a call into the owner to inquire about its status.  

I do like the idea of switching the pressure pump power off at night or when off the boat and I plan to incorporate that into my routine.

Thanks for the discussion.  I'll post as soon as I hear from Borel Mfg.

Gary S. Silver, M.D. (retired)
Amel SM 2000 Hull #  335
Puerto Del Rey Marina, Puerto Rico