Well, I suppose the commercial aspects could be an issue, but just the issues of pond maintenance and fish seems like fair game to me. If part of your reason for the koi is a local attraction, then some ground cover would need to tolerate foot traffic.

Since your pond is natural and the ground should be moist, that should allow more selection. I would like to recommend Sweet Flag as a partial pond edger. One clump can be divided into 20 or more plants etc. You couldn't walk on it but it grows very well and could define areas. Actually, it might grow too well in SC.

I would love Sweet Flag and currently have none. Last year was the first year our pond was 'up and in operation' so to speak and we were able to add a lot of plants, pond plants and marginals and will add many more this year and I hope to locate Sweet Flag somewhere, preferrably from a ponder willing to trade for something I have! Thank you for the suggestion.