For a duck flighting pool, we recommend that you have some deep areas. While it is true that mallard only normally feed in 40cm of water or less, it pays to have some areas at least 120cm deep to prevent the whole pool being choked with reeds and rushes. So, extensive shallows are important, but some deeper water is valuable too. It may also mean that diving ducks like tufted and pochard find your pool attractive too.
As far as fish are concerned, water up to 120cm is probably ideal for species like carp and tench. Trout need water that does not get too hot in summer, and some areas up to 2.5 to 3 metres deep will be needed.
A note of caution should be added about combining fish and ducks; namely that fish usually compete quite strongly with ducklings for insects and other invertibrate foods. This can lead to poor duckling survival, although a creep feeder can help.