We went to Waterbom particularly for the Flowrider which my son had done in Cairns at the local swimming pool on a previous trip.
You can only book this experience once inside the park. Go straight there, as they get nooked out. Every session is 30 min for 125k or 200k for 1 hour. Maximum of 10 people on each side of the flow rider which us split down the middle by a blow up partition. People queue up and take turns. Once you fall, you are off and join the back if the queue. It is very popular. The instructors are great. They have boogieboards and wake board type boards. It is the opposite of surfing, so non surfers will do better here, as you put your entire weight in your back foot like snowboarding which is counter intuitive for surfers. 30 min are not long enough to get he hang of it but long enough to have some fun. Careful on how you fall (and you will fall a lot), my son ended up with a very sore neck and back afterwards.