Heated Sea Bathing on Yuge Island
Although this is not an onsen, it really should be considered a great one! The Shionoyu, Thalasso Therapy Spa on the Yuge Island is a quick ferry ride across from Innoshima. It is only 300 meters from the ferry port, is co-ed, and requires a swimsuit (which you can rent).
This is a unique spa in that the water used is from the Seto Inland Sea. Yes, I tried tasting all of the water in the pools and they were quite salty! Inside are some simple locker rooms, bathrooms, showers, a "walking" only swimming pool, heated and cold tubs, a cave like heated spa area, a non-salt water Jacuzzi, and a steam room (also salty). Outside is another heated salt water pool that has a brilliant view (picture below).
The staff spoke a little English, but many signs were also translated for tourists. Entry is 520 yen and the swim suit rental was ~200 yen.