sunk fiberglass boats "erode" away over time.
I am unsure of how... but I've seen them in assorted stages washed up out of the local lakes when they flooded or exposed when the water got very low.
One I should have a pic of somewhere... was down to just very thin glass on transom and part of the keel. That was all that was left.
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But yes, the pics above resemble shotgun damage.