We dug a small natural pond for our duck, as the soil was clay I had assumed it would be ok and not need a liner. The first year the water level changed dramatically with the seasons and we were considering puddling it, stuff for in the way as it always does, any after years 2 and 3 the level stays pretty constant. I put in a spill way pipe so the overflow runs into our drainage ditch in heavy rain but there is only minimal level drops in the summer now. Nature found the way!
The only think I would say is our water never looks really clean, no doubt this is partly the ducks, but also the clay side. This year I will look at some plants to clean it up a bit and if that doesnt work a circulation pump maybe.
Good luck with your pond