How much water?

A small slide needs a bath-full of water every minute. A big raft ride needs a swimming pool's worth every few minutes!Water is the lubricant between skin (or tube) and gelcoat; without it friction would stop or burn a rider within metres. It also carries the rider like a current. Gravity supplies the energy, friction and drag remove it, and banked walls supply the centripetal force that keeps a rider turning instead of flying straight - which is why a curve's wall rises higher the faster the design speed.