According to Boat / US and their insurance pamphlet, the reason for boats
sinking on open water a


Reason
Percentage
Taking Water Over the Gunwales: 30%
Leaks at Thru-hulls: 18%
Leaks at Raw water Cooling System/Exhaust: 12%
Drain Plug Missing: 12%
Navigation Error (Grounding): 10%
Boat Construction (Hull Split Open): 6%
Leaks at Outdrive Boots: 4%
Struck Floating Debris: 4%
Other:
4%

I was surprised that leaks at Outdrive Boots only accounted for 4%.