Ryuujin no Yu is a lovely spot to watch the sun set over the Pacific, and one of Okinawa's rare onsens, as this island chain isn't blessed with the natural geologic features that make for good onsens, nor is onsen bathing part of the Okinawan cultural heritage. (Okinawan culture is distinct from that of mainland Japan.) A large spa-sauna-onsen complex, there are a variety of indoor and outdoor baths to choose from, as well as the non-water options (steam room, massage, etc.).
Getting there requires getting to Senaga Island, best reached by car.