Expensive electronic devices that send sound waves to scare away unwanted ducks exist, but should only be used as a last resort. A good rousing scare of your own will temporarily rid your property of unwanted ducks, while a dog will scare them away on a daily basis. Ducks are docile creatures, but they can also be a nuisance: Scaring them away can be accomplished with a few warning shouts and motions.
Approach the ducks and shout loudly and wave your arms back and forth in a jumping jack formation until the ducks fly or swim away.
Allow your dog to give the ducks a run for their money. It will be best to keep the dog on a leash at first, until you know how the ducks will respond; You do not want your dog coming into contact with the ducks for his own safety -- and legally this may not be OK, especially for protected species.
Throw small stones around ducks, never throwing one at a duck, to scare them away.
Purchase a sonic bird and duck repellent on the Internet to scare ducks away using electro-sonic sound waves.