I currently have thirty ducks and twenty-one chickens who are all housed together. Ducks are definitely messy, more so than chickens, but each variety of poultry is perfect to me in their own ways.
Water: Ducks drink/need much, much more water than chickens, and should preferably have the option to swim/bathe, but this does mean some extra work on your part: changing their water, maintaining their pool/pond, etc.
Also, be warned, clean water lasts about five minutes at the most with ducks. 
But you learn to love their messy habits.
Housing: As stated above, ducks definitely need to be locked up in a secure area for the night. Just like with chickens, nesting boxes should be provided for laying hens. If they are free-ranged, I’d also recommend keeping them contained until eggs are laid, or else you may be embarking on an unexpected egg hunt through your yard.
Nutrition: As babies, ducks require more niacin than found in regular chick starter, so niacin supplements are always a great idea. When adults, layer pellets should be given to adult hens with option of oyster shells as added calcium for stronger eggshells. In mixed flocks that include drakes, all flock feed is good to add for needed/extra protein for the boys.
Hen/drake ratio: Males can be rough on the females, so be sure to have at least three hens per drake. However, preferably five hens to one drake.
In regards to free-ranging, I personally do not free-range my ducks without supervision. During the day, aerial predators remain a threat and it would be all too easy for a bald eagle to carry off a duck around here. If you do let them on a pond, one of the biggest threats are snapping turtles. They will eat ducklings and easily injure adults legs.
Hope this helps a bit. If you have any questions, feel free to ask.