goosedragon brings up a great point. Ducks are messy creatures and poop a lot. A lot of people really can't stand it after a while.
If you get ducklings, make sure you have a brooder large enough to get them through the first 6 weeks. Think about where you're going to put it. Take into account ease of cleaning (you may want it near a water source or near a door to outside), where the stink will flow to (it will stink! garages or basements or extra bathrooms can be good places for brooders)
Make sure to have your outside predator proof duck house built before they are ready to go outside so you can get them into it as soon as possible! Figure out where you don't mind poop being and fence them away from areas you don't want poop. For me I don't want poop on my front porch, but allow them to be on our back patio. I just hose it down once or twice a day. I put pea gravel all around it so that I wouldn't have a muddy mess from them dabbling in the water when I rinse it off. Know that they will tear up any bits of plants they can reach. I have some shrubs that they have perfect laze about places for themselves because they tore off all the lower leaves. I don't mind, will you? If so, fence it off!
Anyway, those are some of the shady sides of duck ownership.