I have been very successful in keeping duck in my garden while many tried and failed. One reason is I practice a permaculture non till garden method. So my garden has a deep straw and leave mulch. I do not think ducks could be near as successfully kept in a bare dirt plowed garden. Ducks love to play in the mud and will drill little holes where water is standing on mud and destroy roots . This is not a problem in a heavy mulched garden since there is no mud to play in. Unlike chickens the ducks do not scratch away the mulch.
Another reason I was successful I feel lays in my background. I was a professional dog trainer. I was involved in training and showing horses as well. As a trainer I knew you must take into consideration the animals instinctual behaviors and work with them not against them.
That's what I did with ducks in the garden. I researched why many failed when they attempted to keep ducks in the garden. Their failures became my learning experience. I researched ducks and their instinctual behavior as well as what they prefer in their diet. Plus what makes them happy? I took all these into consideration in my design.
What do YOU want from your ducks? Do you want only pest control, how about eggs, do you want a calm pet duck? The answer to those questions will help you choose the right breed of duck for YOU.
There are many breeds of different sizes and temperament and egg production. I chose Indian Runners they are hilarious very active ducks, fantastic foragers, great layers, flightless but nervous. Always on the look out for danger. As a rule they are not good pets for children that wish to hold and pet them. If you have kids or want a pet for yourself that will allow petting I would suggest welsh harlequin, khaki Campbell or Peking's. They are considered to be very calm which makes good pet ducks. There are different size ducks light, medium and heavy breeds so I chose a light breed reasoning that they would put less stress on the younger plants and produce more manageable amounts of waste. I reasoned that an 8 pound duck will produce twice the waste as a 4 pound duck. So 2 Peking's (heavy breed) would be like having 3 to 4 of my light breed ducks. This is why I chose the light breed. I wanted low fences so I chose a naturally flightless breed. Not all light breed ducks are flightless. Do you want a duck who produces a lot of eggs? The light breed ducks I considered were Indian Runners, Khaki, and Welsh harlequin are excellent egg producer. Of those 3 only the WH is a dual purpose bird used for meat and egg production. It is the best for setting its own eggs as well.
Once you choose your breed you must decide on the sex. So what's important to you eggs or noise level? Male ducks do not actually quack loudly they make a lower whisper quack like sound. Two male ducks can live happily raised together. While if you chose to have 3 ducks you can't keep 2 males with only 1 female. She can not survive sexual advances from both. So you must chose either 3 hens or 1 drake and 2 hens.
Keeping ducks in the garden takes a little management. The first rule is keep the flock small. My garden is 53 by 28 feet which supports only 2 adult ducks. They just produce too much waste and would over graze if your flocks too big. But you must have at least 2 a lone duck will not be happy.
Ducks must have drinking water deep enough to duck their bill in past their eyes to physically keep them clean. So they can't just drink from a shallow chicken water bowl. They wash down their food. So if they are eating pests in a part of the garden that has no water near they must leave the area to get water. They tend to work the area more thoroughly when water is near. So I scatter water stations to encourage complete coverage of the garden. If you begin to have an area where the insects begin to multiply then remove all stations but the one in that area and they will spend most of their time working that area. I just use empty coffee can as water stations.
Small seedlings being stomped was one problem some had. So when I plant new seeds I put a little fence around that area with a vinyl netting fence using step in fence post removing it as soon as the plants develop some size. If the plant is a green the duck particularly enjoy I wait until it is about 6 inches high. Until the plant has enough roots to anchor the plant if the duck picks a leave. I do not grow really tender greens like lettuce in the garden because I was told they love it. In the spring when I plant the whole garden with new seedling I will not allow the ducks in the garden until the seedlings are not so vulnerable.
They love to chew on plants while they swim. So I leave a buffer space between the plants they like and their pool. I once put a kiddie pool right next to a pepper plant. While the ducks swam and floated they munched at the same time. I lost a lot of leaves but the plant recovered quickly. So I changed MY behavior I did not try to change the ducks natural urge to munch while swimming. I did begin to throw small bite size pieces of Swiss chard in their pool a few times a week. That made them very excited and happy.
The ducks prefer insects and slugs over plants. They do not eat plants until the bugs are scarce. Once fall and cool weather arrived they began to munch on old bean leaves since the insect population declined. I just had to increase their feed to solve that Issue. I grew swish chard, beans, strawberries, sweet potatoes , potatoes, broccoli, cauliflower, cabbage, kohlrabi, and beets this year in my duck patrolled garden. I did not lose any whole plants. Ducks do enjoy their greens so I increase the amount I planted considering it as growing my own duck food. They munched on them as they pleased but if I had planted only a few of these I am sure I would have lost plants. I also let one cherry tomato grow across the ground so the ducks could just help themselves. Since small duck size tomatoes were available they didn't bother my more difficult to reach large tomatoes. To be successful with ducks in your garden you must manage them taking into account what they like and provide them with it. Since ducks eat greens and tomatoes I planted it for them. If I hadn't they would have chewed on plants I didn't want them to. The people that weren't successful just planted their garden in the manner they always had without taking the ducks needs into account. There are also poisonous plants to ducks like potatoes and tomato leaves. So if these are the only plants available to them there would be trouble. But since my garden was diverse they had lots of healthy choices.
The only part of my system now I am not satisfied with is their pool. But it is correct for a future duckponics project that I will be adding. I started out with a large stock tank pool and switched to a smaller IBC tote cut down to 100 gallon size. I do enjoy watching their joy while they dive and swim in their pool. But the truth is they can exist without a large pool. Your options can be the small tubs sold at Lowes used to mix concrete in. You could put 2 or more in the garden moving them around daily dumping the water in a new section each day. This would be an efficient method to provide their bathing water. If you want to give them more a kiddie pool works fine. The draw backs to this method is it is heavy to pick up and dump and its a lot of water to dump in the same area at one time. You can move it around the garden daily. The reason my permanent pool is causing me a problem is the sediment settles on the flat bottom pools I have used (stock tank and IBC tote) I want an easier pool to clean. Once I have drained the pool I must rinse and rinse to get the sentiment out. I have come to the conclusion that a bath tub would be the best option since it has a slopped bottom made to drain more efficiently. Since I use all salvage materials I have already began to keep my eye open for a used deep bath tub maybe a claw footed tub. Or a garden tub (haha I just got that) A hot tub may be an option that I haven't investigated it yet. I am not familiar with how they drain. I have seen many on Greg list for free with nice attractive wooden sides.
I hope you find the record of my success and failure helpful. Learn from my experience as I did from others who published their attempts with Ducks in the garden.