No, ducks don't have teeth. Ducks have bills made of keratin, the same protein that makes up our fingernails and hair, but no bird today has actual teeth. Prehistoric goose-like birds called Darsornis had sharp, bony, tooth-like structures on their bills, but this feature was lost as the birds evolved, likely due to dietary changes and to reduce their body weight and make flying easier.
Some ducks may look like they have small teeth because their bills are serrated. The serration allows the ducks to strain away the water from aquatic vegetation they eat. Some ducks may also use the ridges on their bills to help them catch and hold on to small fish.
Ducks swallow their food whole. Instead of using teeth to breakdown the plants, seeds, insects or fish that they eat, ducks (and birds in general, some reptiles, crocodiles, fish, and even worms) possess an organ called a gizzard. The gizzard is part of a bird's digestive system, located between the bird's true stomach and intestines. The gizzard is a strong muscular sack which performs the mechanical work of grinding the food so that it is easier for the animal to digest. Ducks swallow grit (small pieces of gravel or stones) which helps to breakdown the food while being ground inside the gizzard.