I consulted with my ducks. They tell me that I should not use soap. However, they also insist that's it's not a bath unless you seriously agitate the water and splash it high into the air.
Seriously, so many of the ducks were doing this in the creek, that my friend and his enormous camera, had to get in on the fun. Two of the Muscovy girls decided at one point to have an argument in the creek. Normally, I try to intervene if birds are fighting, but I decided I'd rather have my shower in the house and just waited in the hope they'd quit before I was forced to intervene.
Animals seem to manage fine without soap, so I figure I'm an animal and try to restrict my use to the minimum.