Depending on where one boats the water can be muddy with suspended solids in it. For example the Fraser River dumps tons of mud and debris into the Strait of Georgia turning the water into a bit of a milkshake. This stuff easily gets through external strainers and the internal sea strainer, too. But on engine shutdown the muddy water in the strainer stops moving through it and instead just sits. At that point the suspended muck starts settling out to the bottom of the strainer where it can compact itself into a layer.
And depending on where one boats, operating in shallow water can stir up stuff some of which can be little solid bits and they, too, can end up being caught and held by the sea strainer.
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