noun
water police; coast guard
Refers to the police force responsible for waterways, harbors, and coastal areas. In Japan, this is often the Japan Coast Guard (海上保安庁), but the term can also refer to local water police units.
水上警察が港のパトロールを行っている。
The water police are patrolling the harbor.
海上保安庁 is the official name of the Japan Coast Guard, a national organization. 水上警察 is a more general term for water police, which can include local units.
Compound of 水上 (on the water) and 警察 (police). The term is straightforward and transparent.