First, pick a breed of duck. Or breeds. May I recommend Cayugas? All black, medium to large ducks, suitable for meat (but mine will never be a meal), pretty broody, good layers, and one of the more quiet breeds of Mallard-derived ducks.
If you are raising ducklings, they can eat medicated chick starter no matter what anybody tells you otherwise. (Old stories from long ago persisit.). Whether you feed them medicated feed or non-medicated, add some Brewer's Yeast powder over it ever day, because baby ducks need more niacin than what is available in chick starter. There are waterfowl feeds but they are pricey.
Ducks need deep waterers, deep enough for them to dunk the or bills to clear their nares (nostrils). They will splash their water. They bill feed and shavings and dirt into their waterers.
Ducks are happy with mud puddles, or kitchen dishpans full of water; they don't NEED water for swimming. But why deny them?
Your Koi pond will be destroyed by ducks unless you fence off a section for plants and places for the Koi to hide AND GET AWAY. The ducks will be quite happy with your kidney shaped koi pond.
Ducks grow at an amazingly fast rate, compared to chickens. Chicks put all their growing energy into feathering out, but ducks stay fuzzy for weeks while bulking up and growing so fast you can see it the next day. And then they start growing their hard feathers.
My Cayugas started laying about six months old.
