If you build an earth pond and stock it with koi, you will not get nice clear water. Koi love to root and dig in the mud. They will keep the water pretty well churned up. Most of the koi hobbiests that do use mud (or earth) ponds, only keep the koi in the ponds part of the year, then move them to liner or concrete ponds for the remainder of the year. The ponds need maintenance to repair damage caused by koi, they need liming to kill off parasites, and the fish need attention to remove parasites. If you are new to koi and want to enjoy seeing them, you would be much better off building a well designed koi pond. Go to the AKCA (link below) and check out their book store. They offer several very nice books on building koi ponds and filters. Also, study study study before you dig the first shovel of dirt. There is a lot to learn and you really will be much happier if you learn it all BEFORE you dig, rather than after you have already installed the pond. Nothing worse than investing a lot of time and money in a pond and then wishing you had done something differently.
AKCA - Associated Koi Clubs of America - Welcome To The