In some cases, it is possible that the mother duck moved them out of her nest, or may have been stolen by other ducks. Additionally, they may have been stolen by predators like mustelids, snakes, raccoons, and even humans.
Egg-burying is a routine for ducks. They do this for safety and protection against predators like bugs, insects, and larger ones like raccoons, who target the eggs and gulp them down their throats within minutes.
Ducks can lay eggs in water, but only in rare instances. As afore-mentioned, ducks usually nest on a patch of dry land near water, most preferably a position with some vegetation cover that helps camouflage and hide the nest.