My wife and I started raising ducks last year, so we are still learning, but we have found that yes, ducks are easier on the garden...depending on your gardening and management style. Ducks will destroy anything leafy and succulent, and they can destroy alliums merely by trampling. We had a tremendous slug problem in 2010, so in 2011 we ran ducks through the garden area from the time they could be outside until planting. We had almost zero slug damage all year! Then, we waited until the end of the season to run them in the garden again. They ate low hanging fruit (mostly gooseberries) and cleaned up lots of vegetation...vegetable and weed alike, but the only damage they really did was to our beds of garlic and onions, and that entirely from trampling. The breeds we have are khaki campbells and Welsh Harlequin. The Welsh Harlequins are marginally larger than the khakis, but both are active foragers and excellent egg layers. The WH are beautiful though, please look into them as they are a critically rare breed in this country as well.
I would argue than both khakis and WH are too small to effectively use for meat, I got tired of killing, dressing, and plucking or skinning for the amount of meat I got. So we have geese for meat now, and they are a beautiful and comical addition to our flock, and do a great job of keeping weeds down (just make sure you give the comfrey a chance to recover! They are only animals I have ever seen capable of killing comfrey!)
Ponds: we used masonry/mortar mixing tubs. Super cheap, not tacky colors like a kiddie pool, and sometimes I borrow them from our birds to use it to mix thinset or mortar. I also dug a pond and lined it with that rubber pond liner stuff. I made a gravity drain for it so I can drain it for the winter, and to take off the incredibly nutrient rich water...Geoff Lawton has a similar system, gravity fed and all that, except he uses an old bathtub. Smart. The ducks do not NEED water, but without it they will not preen the same way and I can't imagine they will be nearly as happy. They splash more than a bunch of children! At one week old they were swimming underwater and making a mess. Also, whenever a puddle shows up they enjoy it until it is drilled with holes and a mucky mess.
Lighter breeds of ducks do not NEED to breed in water, however I have always given water, and I have never seen them mate on dry land, however I gave my mother a trio of khakis, and they will occasionally mate out of water, even given the choice.
