Covering 20 acres and stretching half a mile, the swimming hole at Chile’s San Alfonso del Mar resort is a 115-foot-deep, 66-milliongallon monster. (An Olympic-size pool is “only” 164 feet long, 6 feet 7 inches deep, 660,000 gallons.) Built for $3.5 million by the Chilean firm Crystal Lagoons, which aims to open an even vaster...