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I agree Donacia semicuprea is the most likely. The dimples in the middle of the thorax are a good indication. D. versicolorea has these too, but its hind legs are much longer and stouter.

1 December is a strange time of year for a Donacia to be out and about. They pass the winter as adults submerged at the roots of their larval foodplant, which for D. semicuprea is Glyceria maxima. Had the canal been recently dredged, which might have disturbed the roots?

John Bratton