I see that Wakayama-ken is slightly under-represented in the onsen list and I would like to remedy that!
Kumano is the ancient name of a region in the south of the Kii peninsula, and is also the site (split between Wakayama-ken, Mie-ken, and Nara-ken) of the World Heritage "sacred sites and pilgrimage routes of the Kii mountain range". It is a mountainous rural area, and isolated, quiet onsen dot the landscape.
In Tanabe city (about 2h south of Osaka):
- Yunomine onsen has a history of thousands of years as a part of the Kumano Kodo pilgrimage route.
- Kawayu is an onsen coming straight out of a river bed : it is free and you can dig your own bath out of the rocky river banks (the water bubbles right out of the rocky banks and soon your bath fills with hot water).
- Ryujin onsen is one of the 3 "Bijin-no-yu" in Japan and its water is smooth and silky to the touch. All ryokans along the river offer osen baths and most of them have rotemburo baths hanging over the river, with the forest
- Tanabe onsen offers a view of the sea
- Wataraze onsen has the biggest rotenburo in the western part of Japan.
You can find more information on those onsen, as well as on lodging and bus schedules in English on the Tanabe City Kumano Tourism Bureau
homepage : www.tb-kumano.jp/en/index.html.
Also close by are Shirahama (it offers white sandy beaches and natural rotenburo in the rocky cliffside by the sea) and Katsuura (site of the Nachi Grand Shrine and many rotenburo facing the sea).
For more information, see the English homepage of the Wakayama prefecture
Let me know should you have any questions!