What about those big bathtub-type one-piece pools?

Not bad if you live in the right spot; with nice, sandy soil. They do tend to be very small, because they have to get under bridges and telephone poles, and the transportation difficulties raise the price as you live farther and farther from the factory. The finish should last a long time, but it still won’t last forever. Figure maintenance costs more like a vinyl-linered pool. Don’t ever leave it empty during the winter: a rising water table (which tends to happen in spring) could pop it right out of the ground like a big boat. Putting it back could cost more than the original installation.