Yes people swim in koi ponds. A recirculating koi pond can be an infection problem especially with a lot of fish. If we swim in a lake or stream, we never swim through the same water twice. The waste flows away. Swim in a recirculating koi pond and waste builds up even with filter cleaning. We can’t see the microbes because they are so small but they are there in the billions. And it’s not just parasitic bacteria. It’s opportunistic bacteria like a aeromonas and pseudomonas that populate falling leaves and seeds from trees.
So keep the fish population low and feed appropriately to the fish weight. Fish food feeds bacteria too. And never wear contact lenses in the pond water. There is an amoeba in lakes that can get under the lenses onthe cornea and erode It irreparably. I got a pseudomonas infection in my index finger because I had a hangnail, and it almost cost me my finger. I cleaned my filter without gloves. Shower thoroughly after entering the pond paying attention to existing sores.