Arusasu is Sai's harbour-side community centre and the place most visitors end up using, even if only briefly. It holds local information, simple restaurants, and the practical arrangements for boat trips along the coast when services are running.
Do not expect a polished attraction. Its value is that it plugs you into a remote village with limited tourist infrastructure. Stop here before trying to reach Hotoke-ga-ura by sea, and use it as a weather check and lunch stop rather than just passing through.