We lined our duck pond with concrete, which of course makes it a bit permanent, but that works for us. It can get green and smelly so we run an overflow pipe from one of the water butts, which washes it through - the 'beach' is lower than the sides, so the manky water runs out that way. Obviously that doesn't work in a drought, so we sometimes scoop all the water out with buckets in summer and use it to speed up the compost heap, or directly around fruit trees and bushes - great stuff. Ducks will tend to pull up any plants in the pond itself, but you can plant lots around the edge.
In fact we don't have ducks any more, so the pond is reverting to nature a bit, in spite of the concrete lining - a bit of silt has settled on the bottom and it now has yellow flag and water mint in. The hens use it for drinking, and the frogs and toads breed in it.
For depth, I would suggest it needs to be deep enough for the ducks and geese to reach the bottom when they upend themselves.