direction of a current; course of a ship
Nautical term referring to the direction a current flows or the heading a ship takes. Often used in navigation contexts.
Steer the ship by reading the direction of the current.
この海域では水先が変わりやすい。
In this sea area, the course of the ship changes easily.
Compound of 水 (water) and 先 (tip, direction). The exact historical derivation is uncertain, but the term is conventionally associated with nautical navigation.