Why not a thin concrete-and-wire-mesh shell?
When I was a kid I helped one of my Dad's friends build a large-ish sailboat out of this. It was pretty easy to do. Seems like making a pool liner should be even easier as you're not concerned with keeping to an efficient-through-the-water shape (I remember having all these plywood cutouts that the hull shape had to exactly fit) and weight isn't an issue for you either - so you won't have to worry about the skin being too thick.
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