Don't bother. For a number of reasons.

As was stated before, Ball Pythons are not a species of swimmers. They will do it, but they won't do it often, and soaking is a potential sign of mites, as well as low-humidity.
Ball Pythons are most comfortable hiding, and with something like a waterfall, it's going to be wasted effort as the most they will use of that is the cave, and even then the snake is unlikely to use it if it has water in it.
Ball pythons tend to react to water by trying to get out of said water and into something like a rodent burrow, or other place they will be protected from predators.

That's the reasons not to do it because it's effectively a waste of time and effort. Here are the reasons to not do it because of potential harm.

Ball Pythons aren't the most graceful of climbers, and will often just plop to the ground, potentially hurting themselves.
With that in mind, the snake may well climb up the waterfall, and plop right into it, which could lead to wet substrate and potential mold issues.
That said, it could also lead to injury of the snake from falling.
If you are not cautious as to how it is built, the snake could drown.
It's probably going to poop and pee in the water if it does use it, potentially damaging the pump system.
It's something else to clean, which means it is going to just increase the effort you spend keeping the snake in a clean environment.

I would not recommend giving your ball python a waterfall, or any such system. However, if you wish to have decorative waterfalls, there are a few lizard species that would highly appreciate that, as well as species of snakes and lizards which require much higher humidity, which would appreciate that.