Koganezaki Furofushi Onsen

Two outdoor pools sit right at the edge of the Sea of Japan, one mixed (konyoku, 混浴), one women-only, with the waves close enough that you feel like you're floating in the ocean. The water is a reddish-brown chloride spring, source-fed and continuously flowing, warm enough to stay in through Aomori winters. Sunsets over the sea are visible only from the mixed pool, and the spot is listed among Japan's 100 famous sunset viewpoints.