Hirauchi Kaichu Onsen 平内海中温泉 is a tidal hot pool. It boasts very hot water (depending on the tide) with pleasant views out to sea, as well as waves lapping right next to you as you soak.
There are two outdoor onsen (rotenburo) situated at Yudomari 湯泊 and Hirauchi 平内 on the south-west of the island.

Yudomari Onsen 湯泊温泉 is tepid and the main pool sits back from the sea. Further along the path is a more private pool, beside the water's edge.