You probably aren't going to like what I have to say, but an ornamental pond and ducks are a bad combinations. The ducks will trash the plants, foul the water, and even kill smaller fish if they can get them.
Its better to keep them out of your koi pond, IMHO, and give them something like a kiddie pool you can easily keep clean by dumping and rinsing.
Of course, this depends on the size of the pond and the number of ducks as well == a few ducks on a very, very large pond with intense filtration, a lot of plants, and good cover for the fish might not cause unacceptable levels of damage. OTOH 3 or 4 ducks on your typical 8 to 10 foot diameter suburban backyard pond will trash it pretty well in about 2 days.