I don't see why a person would have to keep putting chemicals in a pool. And I think a pool would make a lousy compost pile or mound bed. (turning it would be a pain in the butt

What if you used it as an
aquaculture project? There are plenty of good edibles that will grow in such a situation. In Portland my business partner Steven Ganister and I took on converting a pool to a natural pool. The trick was that we couldn't easily add enough marsh area for it to work, because of the topography. Couldn't even drain the pool because it was on a somewhat unstable slope. Basically we built a filter of river rock and geogrid in the shallow end, and put an intake manifold in the top level of the rock terrace. Just that improved the water quality, but then we connected the intake manifold to a pond pump and piped the water up to a waterfall we built, and a series of marshes, streams and waterfalls. The client wanted "over the top", so only 60% or so of the plants are edible or medicinal, the rest are just gorgeous and/or habitat for hummingbirds, beneficials and pollenators. We did have some problems with the (Gasp!) invasive BULLFROGS moving in, but a 10 yr.-old boy who was visiting happily caught the tadpoles (which were eating the water lilies) and sold them to us for a penny each. We turned the tadpoles over to Steven's chicken flock at the ecovillage, and the chickens thought tadpoles rare fun indeed. So: the water is very clean, the fish are happy, and you can eat the wapato (Sagittaria latifolia- the best tasting Sagittaria, local indigenous crop) and the variegated water celery, and lots more things. And there's still room to take a dip. In zone 9 you could grow Lotus too... and papyrus, also edible. We haven't tried floating veggie beds, but why not? If filling the pool in, I would use round gravel (river rock) because that would allow more water storage than "clean fill" would. Fill might have things you didn't want... In any emergency you'd have lots of good water.