Your right about the walls of the swimming area. However the water is usually so clean that you can see deep, so it would be worthwhile using a natural material like big boulders, sandstone block, or even rot resistant timbers. I think the reason the deep pit, steep walls, is that this area is deep enough for swimming but it helps separate that margin of plants and swimming area. If there was a bit of a higher border of the walls( where plants meets swim area), the soils should not so easily wash/erode into the deep area, helping longevity.

Fish may be detrimental to this kind of pond. I would research a lot on this.

If needed, couldn't a UV filter could be added later thru a pumping sitituation, like in a water fall setting? A skimmer might be helpfull with aesthetics and eliminate some regular chores.