I have 6 ducks, and one of my female ducks gets on top of my other female and holds her by her neck. They’re all only about 4 months old. Well today i had let them play in some water inside (they live in my garage because we haven’t gotten to build them a coop outside yet since all the snow) and she was holding my one duck under water by her neck. Can someone help me out with why she’s doing this?
It's mating behavior. In the absence of drakes, ducks will attempt to mate one another. Swimming/bathing water is like an aphrodisiac to ducks.

