I am reading here about concerns how to choose a special builder and what are the cost when deciding to build a swimming pool. That is really not necessary, it can be built by everybody with help of local village mason - chang pun. And for not so big money.
8 years ago when I had discovered that behind the ruined house (10 years abandoned) - we were lucky to find and acquired from a bank – there is a space allowing quite a long pool, the realization had not been postponed for a long. To neatly dig out 12x3m, some 1.5m deep had taken half a day to a skilled backhoe. Next day a rebar mesh 1 hun (3mm) 20x20cm had been laid in the bottom, the sides 0.5m risen for future connection with walls. A smaller CPAC truck was invited with a foundation cement mixture (the lowest quality 1,400B/m3) and some 10cm layer had been poured on the bottom.