Nowadays a professional hydrologist is who you'd contract for the task. They will do a geologic survey and fracture trace analysis using data from satellite imagery, the USGS and related state agencies in order to identify fissures within the underground rock formations that are most likely to hold a suitably sized pocket of water with good recovery. Then they might conduct electrical resistivity tomography to directly measure and image the density of subsurface material to identify water pockets and narrow down good drilling spots. It's a far cry from the sort of thing that might be characterized as dowsing.

I've also had experience installing a catch box (there's another term for it that escapes me right now) in a creek and running miles of pipe always with a slow decent to gravity feed several holding tanks. A word of warning, bears are curious about the sound of running water through pvc pipe, so bury that pipe...