Mallards are generalist foragers and will eat a wide variety of food. They don’t dive, but dabble to feed, tipping forward in the water to eat seeds and aquatic vegetation. Cornell’s All about birds
Anatidae (ducks, geese, swans) are generally divided into dabblers and divers — and never the twain shall meet. For years I have watched Mallards dabble in the shallows (they tip up and scouring the bottom with their bills) and wondered why they seemed incapable of learning to dive from the mergansers they see around them.
Today (with no mergansers present) I watched Mallards dive. Now, they might have been driven to desperation by the snow covered ground, yet they did dive. (What they retrieved is unclear.) I did find some sources that allowed that mallards will occasionally dive for food, but others that said they did not do so.
A Mallard drake starts its dive.
What the Mallard retrieved was unclear, yet it did dive for comestibles.