Hirauchi Kaichu Onsen is a tidal hot spring that emerges from rocks on the east coast shoreline, accessible only in the two hours around low tide when the sea retreats to reveal the pools. The hot spring water mixes with seawater in natural rock basins just above the waterline, and you can sit in the warm water with the Pacific stretching out in front of you.
The experience is rough-and-ready — there are no changing rooms or facilities, just the rocks and the sea. Bathing suits are required and the pools fill with sightseers and soakers on clear days. The onsen is free. Check a tide chart before going; the pools vanish completely at high tide. The nearby hamlet of Hirauchi has a couple of small guesthouses and minshuku, making it a quieter alternative base to Miyanoura for those who want to explore the east coast.