Kumano's rivers, due to their clear and calm waters, are perfect for taking a dip or practicing water sports. During my stay in the Kumano area I had the chance to do some kayaking with a group of people I had met there. We had three kayak boats and two inflatable paddle boards. The children in the group seemed to particularly enjoy the latter. The river trip lasted for about four hours but it felt much briefer. Needless to say, it was real fun. If you get bored in summer in Kumano, it's because you want to!
More details on booking kayaking and other water sports in the area can be found on the Kumano Tourism Bureau website.
Can you share some details on how/where were you able to rent these boats ? I'll be with friends in Kumano this summer but boat rental agencies seems to accept fluent japanese speaker only. We'd rather paddle down the river than take a traditionnal "motor" boat.