It is not water, but treasures which they profess to find by some hidden kind of rhabdomancy.
Hazel-rods were used to "divine" for water and minerals by professors of an art which received the crack-jaw title of rhabdomancy.
Sir Thomas Browne also speaks of a 'strange kind of exploration and peculiar way of rhabdomancy' used in mineral discoveries.
rhabdomancy, a species of divination by means of a hazel rod to trace the presence of minerals or metals under ground.
1640s, "use of divining rod" (especially to discover ores or underground water), from Greek rhabdos "rod, wand; magic wand; fishing rod; spear-shaft; a staff of office; a rod for chastisement; twig, stick" + manteia "divination, oracle" (see -mancy). Greek rhabdos is from PIE *wer-, base of roots meaning "to turn, bend" (cf. Lithuanian virbas "twig, branch, scion, rod," Latin verbena "leaves and branches of laurel"); see warp (v.); the Greek noun was used to represent Roman fasces. Related: Rhabdomantic