Below are some tips from our own experience water rafting on Ayung River.  Feel free to jump in and add any advice to the below :p:

• we paid around 35 USD per person for the ride (possible to find even lower prices); this price included a person picking us up from the hotel and back, the boat ride and something to eat after the boat ride; beer is available for buying during the boat ride at a stop on the way and at the end of the boat ride;
• you can take a shower at the end of the boat ride; they give you a towel or you can bring your own;
• wear a bathing suit and some comfortable clothes like short pants and a t-shirt; you will get wet, very wet;
• flip flops are a good choice; in the boat you will be barefoot and your shoes (or flip flops :p) will go into the bag of the boat leader;
• phones, cameras, money – take as less as possible; whatever you carry will go into the waterproof bag of the boat leader; you can’t keep it with you in the boat since you will get wet – yeah we said that before :);
• we took pictures and filmed with a GoPro camera;
• listen carefully to the instructions of the boat leader/guide and do as he says during your trip if you want to have a safe ride;
• go with the flow, be carried away, enjoy the ride!