I collect roof runoff water in rainbarrels, any silt settles in the barrel, the overflow gets connected to a drain hose and runs 100 feet to my pool and fills it (gravity) - I do this in the spring since trucking water in gets expensive.
This year, all I used on the pool is bleach and about 10 pucks inside the stairs over the season, even my CYA stayed at the right levels - hey, maybe I got lucky and our ACID rain is actually CYanuric ACID rain!

WRT the guy who built the plywood pool, I've seen numerous pools like that, real simple to make (marine grade plywood), very sturdy, use a pond liner if you want, and a garage sale pump and filter - hey. some cultures even line the bed of their broken pickup truck for a splash pool!
A while ago, a newer member on teh forum actually planned on a plywood pool and was asking advice here, I don't think any of us were any help, a project like that is usually "fly by the seat of your pants".
There's a neighbour of mine who (believe it or not) built his basement walls using marine grade plywood - I've seen it with my own eyes, 30 years old - like new - I asked if when he built his house he had it inspected, he said yes, and the inspector argued that it wasn't allowed, he pulled out the code book, and showed the inspector exactly where it was allowed, he left very red in th e face after giving the stamp of approval.
Just because it isn't the norm, doesn't make it wrong. I look at everything with an open mind, you tend to learn more that way.