No better time to take to the water, than during the Green Season in Costa Rica! The waters are running high, which makes for much better rafting. The pre-trip research found that there was one main river close to Tamarindo with class 3 and 4 rapids. For better white water rafting, and a more notable area for white water rafting, head to Arenal.
Getting a shuttle to the launch point was the only option I found for white water rafting. We had our own rented vehicle, so we were willing to drive to any launching destination, but I couldn't find direct access to any white water rafting company where we could park. I'm guessing this is because the shuttle (in our case anyway) is also used to take participants to the river start and pick them up at the finish. If you had your own vehicle, I'm not sure how the rafting company would get you transported, unless you had a member of your group staying with the vehicle. I just checked the price difference from the rafting company and the shuttle company's published price, and it really wasn't that much of a difference. I'd pick the shuttle service for ease of getting around.
Because we were there in the slower, Green Season, booking in advance of being in the country was not necessary, but you do need to book a day or so in advance to schedule the shuttle. Tamarindo Shuttle was very helpful in booking our white water rafting trip, and providing a nice, comfortable ride to the launch point. Our white water rafting experience was provided by Rincon Corobici Rafting and our group of river guides were fun and crazy brothers.
Experiencing the landscape of Costa Rica from the river was fantastic! We heard monkeys and saw lots of lush vegetation. I would recommend a white water rafting while in Costa Rica as one of your adventure activities. Don't be surprised if your raft gets chosen for an extra special splash at the end of the ride (I have a feeling they like to target one raft for a flip)!