平潟港温泉 is a hot spring town in Ibaraki, Japan. The water leaves the ground at 63.4°C, hot. Officially the water is classed as chloride spring. 20 inns take visitors here.
Salty water. The salt leaves a thin film on the skin, so you stay warm well after leaving the bath.
Figures from the town's Japanese Wikipedia article.
Planning a first soak? Read how an onsen stay works, and what to do if you have tattoos.