A skimmer, if sized appropriately, can make a big difference when it comes to surface aesthetics. I am talking about a screen-type system that pulls water from the surface and basically screens out the bigger floaties. Combine that with a water filtering system that filters out the smaller bits and water clarity starts to improve as well. I'm not a expert on the matter by no means, but I have been to BassPro and their water systems are top notch...ducks and all. They have basically created swimming pool clarity without the chlorine.

Your earth bottom pond will not likely get to that level, but given the right budget...improvements can be made.

One thought is to control the duck's diet. Cheep food creates more waste and excessive feeding will amplify that.

There's also plenty of talk here at the forums about Alum treatments for dropping out nutrients in the water column. That might be something to consider. I don't know the first thing about it, but it's worth checking into.

Plants may also help some if the pond is lacking in vegetation.

Fish on!,
Noel