A small naval vessel used for patrol duties, such as coastal surveillance or harbor security.
哨戒艇が領海を巡回している。
A patrol boat is patrolling the territorial waters.
巡視船 is a patrol vessel used by the Japan Coast Guard, while 哨戒艇 is a more general term for a patrol boat, often used in a naval context.
Compound of 哨戒 (patrol, scouting) and 艇 (small boat).