This happened to me once when far offshore, only I found the problem in time; another time on an OPs boat during a delivery.
Since then my practice when on passage: close ALL non-essential through hulls, including the 'maintenance' bilge pump, manually pump the bilge every hour - counting strokes to pump so one can 'see' the development of any problem - each hour and included in the 'deck log'.
Only my high capacity 'emergency' bilge pump with alarm is left on.
Each time I go offshore and prior to departure, I inspect each and every throughull and its 'double clamped' hoses, etc. You only have to have water over the floor boards once to make you a believer in periodic inspections, etc. When offshore its really bad practice to wait until the boat starts to 'struggle' deeper in the water to wait to 'discover' that you have a problem .... when pumping the manual bilge pump on a routine basis will tell you much earlier.
For this sinking, high on my suspicion list would be: dripless shaft seal as when they 'go' they go 'fast and full', an engine that didnt have 'double clamp' connections, etc. on the entire cooling loop, etc. A high water bilge alarm should be required on all boats.
