I have a good size garden, and 8 ducks. I had to fence off most of my garden until every plant and seedling was well established. They love the leaves of some plants - like potatoes and pumpkins, and left other leaves completely alone - tomatoes, raspberries. You can't trust a duck around a head of cabbage at any point.
Last year we were completely overrun with snails. I would go out at night around midnight with a bucket, and gather 50-70 a night for a week straight. I haven't seen a single snail in my garden in 4 months. No joke. No slugs either, and most of my potato bugs are gone too. They have also eaten every single tomato in my garden- they breached the chicken wire fence so many times I gave up and have let them take over all 6 tomato plants. They wait for them to ripen, then eat them all :).
I don't think my ducks would be happy about walking up 3 feet to their house. I know chickens don't mind - but ducks tend to like to stay close to the ground. I have been really impressed with my Runner ducks overall. They eat about 1/2 as much pellets as my other ducks, and they are excellent foragers. And for their petite size, they lay some amazingly large eggs. My two pound Runner lays the equivalent of an extra large chicken egg every single day.
Their water would make a great fertilizer - we have a pond, not a pool, but we put all of their used bedding in the garden, and pump dirty pond water into the grass. We ahven't had to fertilize the grass or the garden once this year. They have also completely eliminated our dandelions. I haven't used an herbicide since we've had the ducks, and I haven't needed to. They will eat any broad leaf weed they can find.
There are downsides - but that depends on your situation. For me - the problem is that my entire yard is manicured and nice. I don't know how else to say it. It is completely landscaped. This is not a livestock yard. It is grass, and landscaped flower beds, with a large pond and waterfall. Adjusting to the amount of poop on every surface of our yard has been the biggest hurdle. Grass, rocks, dirt, walls, every surface gets covered in poop. It's not a huge problem right now, you can hose it off, and it filters down to make a great fertilizer. I am worried about how bad it will get in the winter - when I can't pull out the hose and clean everything. I am considering blocking off part of my yard, so they only have access to the pond and the garden. I'm figuring it out as I go.
Here is my yard and ducks - they are so spoiled and loved!
I just want to say - for how messy, stinky and gross they are - I wouldn't be able to give them up. They are beautiful, intelligent and hilarious little creatures. They eat their weight in bugs, give my family delicious eggs, and are great friends to my kids. They eat all of our leftover or less than perfect fruit and veggies, and turn it into yummy eggs. I can't believe I waited this long before I raised domestic ducks!