My pond is unlined so is more a wildlife wildflower pond, water levels can range from 1.2, to 100mm which is where it is at present with the tops section dry. We used to get a pair of Mallards however the pond is now overgrown with Bob Bean, Marsh Cinquefoil, Common Reed, Broad Leaved Pond Weed, Water Cress, Mash Marigold, Flag Iris & aquatic grasses, with Monkey Flower & mosses on the higher level, with Meadow Sweet round the edges, so there's little clear water.
I'll have to go in at some point & dig out some common Reed before it takes over. Blanket weed & Duck Weed are also a problem as I can't skim them off due to all the plants. The birds love the changing levels, my Tree Sparrows in particular as they look for insects & seeds.
If I has an area of clear water I'm sure ducks would be back.