I've also found a few west side ducks eating seaweed and assorted attached creatures.
The ducks here feed on eelgrass (Vallisneria neotropicalis), milfoil, Najas sp., coontail & other submerged grasses. But they ingest a pile of invertebrates with it. Almost all the ducks we kill, puddlers and divers, will have snails /snail shells in their crops & gizzards right along with the grasses.
We get sea lettuce in the Shoalgrass beds down on Mississippi Sound in the cold months and the redheads & greater scaup feed on it as well.