Have you seen some of the ponds or "pools" that are special for a natural swimming area. They are built with stone edges/wood in the deeper area and the perimeter is shallow with plants that naturally filter the water. There is one guy near me who has been building one for a few years now. What I have seen of them, I would think it's a bit costly to build, but worth it if you like that sort of thing. What maybe you could do is utilize the hole for it's depth, and begin making one of those. Perhaps it will not cover that whole area, and you would fill in the perimeter once you got the "pit" built. Of course this depends on funds available. Probably a liner is used. There has been a few posts on here with people wanting to this, but none that I can remember have come back and shown progress.