The coaster harbour is a freestanding building that must be built within a town's main area. It can have internal rooms. However, its prime purpose is to allow ships to dock that usually can't; any ship/boat can dock to a harbour, even if they can't dock to the town's beach. The building can be locked, but boats may still dock.
The harbour can only be built when an area has access to the sea. If the area is on a lake, a landing-stage is required.