On a clear day, the outdoor baths provide pleasant views (partially blocked by trees) of the waterfront and the restaurant in the hotel is reasonably priced, serving fresh, local food. A nice onsen, but perhaps not worth the journey unless you’re in the Miura area.
Small natural onsen that is part of Hotel Keikyu Aburatubo Kanchoso in Miura, Kanagawa Prefecture. While the hotel itself looks like some sort of run-down motel from the outside, it does provide very good views of Koajiro Bay from the outside bath tubs. The onsen is not all that big, but the hotel’s slightly remote location means that it never gets that crowded, leaving you to enjoy the views in peace.
There are just three tubs: one inside (set at a lower temperature), and two outside, including a one-man Jacuzzi pot. The natural spring water comes from deep under the surrounding seabed, and so has a high salinity and is salty to the taste. The hotel also has a good restaurant which serves local food (in particular “Misaki Maguro”—tuna caught in the region) at reasonable prices (¥1,000-1,500).