Changes in Conditions as Water Levels Change
The challenges presented by a specific rapid can change as the river flow or water level changes. As a general rule of thumb you might assume that more water makes rapids more dangerous and difficult. But some rapids can actually become easier with more water. As the features and challenges of the given rapid sink far enough below the water such that they are washed out and no longer affect the surface. With that said, rapids can also become more challenging with more water.
With the water moving quicker, you will have less time to respond between obstacles. Sometimes that quicker water creates unavoidable waves and holes that previously were easily avoided. At other times the size of standing waves can significantly change, moving from a fun wave you used to just splash through, to a boat munching wave requiring great momentum to punch through it, or better yet avoid entirely. Sometimes, new pathways open up that can either cause new challenges, or provide alternate routes that are easier than the path you normally take at more regular flows.
All of that to say, be aware of changing river conditions.
Rating systems for a specific whitewater river can vary from one local to another. One class river or a specific set of difficult rapids might be easier for a kayaker versus a rafter, or might greatly vary based on water flow. Rafting is a fun but complex sport, and you should be respectful to the level assigned to each river and take the time needed to scout out a rapids water upstream and downstream before attempting it.
Whitewater rafting with a qualified guide and commercial company on Class III, IV, and V rapids is a great and fun experience. While you might get a larger adrenaline rush on the higher classed rapids, the overall amount of fun is relatively the same.