If you want to make it strictly a swimming pond, look at natural swimming ponds. Basically you have to have the same area as the swimming area in natural plants so they can filter the water. https://www.hgtv.com/outdoors/outdoor-spaces/pools-and-water-features/natural-swimming-pools-101

Other than doing that, if you want to have fish in there, don't put Bluegill or Hybrid Bluegill in there, they will nit at swimmers. If you don't mind the nipping, then anything goes.