Unless the whole duckling is bloody or dirty, I'd keep the water away from everywhere but the wound. You don't want to make its' little body work hard to maintain heat as well as fight the injury. I also wouldn't use any soap, just yet, unless you see dirt. Dawn is used more for true baths, and oily cleanups. Blood is water soluble and water shouldn't irritate the injury any more than necessary. Once the injury is cleaned a bit, it will (hopefully) be easier to see and stem the bleeding. Do you have any antibiotic cream handy? That should help, as well.
Ducklings are very resilient. They can really surprise you with what they can put up with and fight through!