Still I think the drake is your culprit. He does look fairly typical of a bedraggled WH - and he's in eclipse here so this is the phase that's easier to tell. Unfortunately with some crosses it's very difficult to tell what the base is. ie...a white layer will nearly always throw white offspring, but those offspring may carry a different color gene and pass it to their young, etc.
But, keep in mind if they free range all day that a wild (mallard) could stop by for a visit... that could also happen. You don't have any other ducks, right? I've had that coloration happen from a few different crossbreeds - even one that according to the calculator was going to give me 75% snowy and 25% runner pattern. (I was working on a new line of harlequin runners at the time, but since abandoned the project and stuck to Show Quality WH)
Ps- your females are really quite light so it's odd they are throwing such dark ducklings. Sorry to say, but If you are breeding to sell you'll need to replace that drake. At least you can still sell this years ducklings as "egg layers" Just not as a specific breed.