No curtain parted on Arm-of-the-Sea Theater’s new production Under Mill Wheels when it premiered last weekend, since it would have had to drape the entirety of Saugerties from battens fixed in the clouds. The watery, kaleidoscopic set served as the focal point of a story that implicated almost two centuries of town history, from Henry Barclay’s purchase of 150 acres of land in 1825 to that pair of summer nights in 2012.