Give up the rivers and go surf!
I was there in November two years ago for a surf trip, I saw some rivers but they didn't have a whole lot of water in them. I believe that november is the end of the rainy season, so there probably won't be much water anyway. We stayed at a surf camp in El Paredon Buena Vista near Sipicate (pacific coast), the surf is constant 6-8 ft swells with a nice beach break. This would be an excellent place to surf kayak if you had a boat, if not, a board works well. The camp has a web site elparedonsurfcamp.tripod.com/. These are some of the nicest people who I have ever met and it is really cheap to stay there. I think they will even pick you up at the Airport (which can be fucking crazy). Good luck!