Sorry for your loss I know that must really get to you. I couldn’t say why you boat sunk but according to Insurance losses approx 90 % of all boats that sink sink at the dock. Usually the most common reasons are tides which can cause small boats to get wedged under the dock and when the tide comes up get swamped, bilge systems, batteries, hoses, and leaks plus rain.
Personally I keep my boat on a trailer 95% of the time and when it is in the water I will check it or I am using it every day.
FellowShip
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Just for the Grins
Official 23’ SF Antique Classic SeaCraft Owner