Yep, they are mating. Girl ducks do it with each other when there is no male present. It is referred to as playing Hop on Top. Do you have pictures of your birds? I'm sure they are lovely! And yes, they mate in water - the head bob is the first step, to make sure they are both on board ;)
Two of my girls will still hop on top of each other even though I have two drakes. Who knows, maybe they don't like boys?