S-N-R, most ducks are omnivarous, filling up on anything available, vegetable and animal, all you mention and more, fishes, mussels, sea urchins etc. The superb canvasback is particularly susceptible to lead poisoning because it strains seeds and sedges out of the bottom mud and, according to Audubon, "in heavily shot-over areas 10 per cent of the ducks may carry shot in their gizzards, as many as 96 shot have been found in a single bird." Our blacks feed on seeds until ice and then blue mussels and periwinkles in shallow salt water. I shoot over more hard bottom than soft.