Breeds that don't fly are pretty much limited to pekin or rouen ducks unless you keep their wings clipped or you encage them. if you choose wing clipping you have to watch them closely so you don't allow enough primary flight feathers to grow back enough that they can fly. They will.
Both the pekin a rouen ducks are bred as meat ducks with short stubby wings. I've seen them try their hardest to fly, it's quite comical and a bit sad.
DUCKS ARE MESSY. They use water to swallow food and sputter their beak in the water to do so adding, god knows how many, pollutants to the water. They eat grass and bugs in good weather requiring very little feeding at times which is a big plus. However, they poop like it's their purpose in life and they may uproot aquatic plants looking for food. Not always, but it happens.
Ducks that can't fly are very much in danger of being killed and possibly eaten by local predators. They are very passive and pretty much just run quacking loudly when alarmed. It's really a very bad form of self-protection. :|
Raccoons, minks, skunks, coyotes, dogs, etc. are all predators that might kill them. My parent's dog killed 2 ducks of mine for no reason after a year and a half of living around them. A large male goose prevented the rest from being slaughtered. Geese are big and evil, don't get them, lol.
Ducks do, as well, like to waddle around. My ducks wandered up to a quarter mile from their quarters. Mine wandered across a state highway, a quarter to half mile from their pen, shelters and pomd, within 2 hours after feeding.
Let me tell you, herding ducks across a state highway is NOT a fun or relaxing job like I thought keeping ducks would be.
They ARE very amusing, cute, comical, friendly, shy, delicious, beautiful little fellows to have around.
If you do keep ducks always remember; One or two will never feel safe, they are flock animals, get at least 3. Males will tear up and rape females to death, always have more females than males. *I can tell you how to tell the difference in different breeds.* They MUST have water available to be able to eat. Like I said, they use water to eat their dry foods. They lay very large delicious eggs if you have females. Rouens even lay blue eggs. They will always be grateful for a pond to splash in though. First bath I gave mine every spring before the fields thawed out the ducks were always like little children given a cake a a puppy and ball pit.