Do I need a liner? (and other scatter brained questions)

We dug a pond that measures ~50'x~80' and varies from 3' to 6' deep, though it was initially dug about a foot or so deeper. It's mainly fed with runoff from the slope above. There is no aeration or circulation aside from the run off and natural convections that might occur via the difference in depth. The heavy clay here held water from the get go.

It's been great in so many ways, but we don't swim in it. We'd really like to change that. We've become enamored with the whole organic pool thing that David Pagan Butler does, but we really don't like the idea of a liner. It doesn't seem like we need it as our pond holds water really well. Purchasing a liner large enough for this pond is also a lot of money for something that brings so little to the table.

So why else might I want a liner?
- Limiting nutrient flow from the clay to the water? Is that point moot since the pond is fed with runoff anyway?
- Would it provide us with a less "mucky" bottom? If this is the main appeal, could I just put a woven geotextile fabric on bottom and lay bank run or stone on it? It's still extra plastic, but for an easier to digest price.

Also of note, we had ducks for a number of years who would spend most of their waking hours in or around the pond. They're gone now, but should I be concerned with whatever microbial life their poop might have introduced into the pond? Would copious aeration get us out of the duck poop bind if it's a problem?

Would the aforementioned aeration lead to an uptick in algae? We have never had any visible algae.

We have LOADS of frogs, tadpoles, salamanders, dragonfly, damselfly, and a small slider turtle that'd I'd hate to disturb unnecessarily. Are any of these signs of healthy water that doesn't require intervention?

We've decided that there's just too much life in the existing pond to drain it, reshape it, and change the pond bottom to various gravels.

The plan is to dig out a new, smaller/deeper pond and not risk the loss of habitat. I'll try to document my semi-educated guess work here for posterity.

I like your new plan.

Have the slopes feed the big pond, and then have the big pond feed your smaller natural swimming pool.

I am envisioning a sand filled trench connecting the big pond and the swimming pool, so that their water levels are the same, and then a small pump sending the water from the pool to the pond for processing/filtration.

If I were you I'd invest in a lab test for water quality in the pond as it stands now. I suspect it would be only $50-$100 or so. If the water looks clear with a wide range of pond creatures as you describe, I suspect it may be fine to swim in as it is. No point fixing it if it isn't broke.

My understanding is that the liner, and the depth that David suggest for swimming ponds is to do with stirring up silt from the bottom of the pond, which will make it cloudy and be more likely to swallow slime and something nasty. You could try a geotextile with a layer of pebbles, or just a thicker layer of pebbles (my preference). It could be done just in the areas that you are likely to be stepping and swimming in.

I'm now building a polytunnel instead of a swimming pond, but am still a bit jealous!