water-finder
/ˈwɔːtəˌfʌɪndə/noun
1A person who uses particular skills or knowledge to find an underground source of water; specifically a person who searches for underground sources of water using a divining rod or similar technique; a water diviner.
2An instrument for locating an underground source of water.
Origin
Mid 17th century; earliest use found in White's Peripateticall Institutions. From water + finder.