If you can get a narrower bucket that comes up to about 2" below the ducks' "chin", they can get their heads in there to drink but won't be able to cimb in to swim. But you always need to keep it topped off.
I personally just use the same 3 gallon rubber bowl for their water. Sure, they splash around in it, but I keep it on top of patio blocks so there's no mud.
If you don't want them to climb in it, place an upside down ceramic bowl or crock in the middle so there's no room to swim but leaves plenty of room around the edge to drink and dunk their heads.