I was a Teaching Assistant for a wildlife techniques class this past semester, here are a couple things the guys that lectured on ducks told us:

18" deep water is one of the most critical points. Mallards and similar ducks can only reach down about 18" to feed.

Ducks prefer small ponds like yours to have a roughly 50/50 mix of vegetated and open water areas.

Most of the small ponds the nearby wildlife refuge uses for holding ducks during hunting season use a flash-board riser system so that they can control water level just by adding and removing 2x4s. In the summer the water is drained and they plant millet. Then they add water and hold it from about 2 weeks before hunting season begins until the ducks leave. Adding water to the millet too early can cause the seeds to rot. They referred to this system as a moist soil unit, however it could just as well be called a pond as many of their moist soil units had deeper areas that never drained and supported a decent number of catfish as well as tons of crawfish.

Every person should have an interest in life - I think I'll go fishing. ~ Thoreau