Why does the serpentine belt come off everytime I drive through a puddle?

Most commonly one of the pulleys is tipped due to a worn bearing but a sticking spring-loaded tensioner can leave the belt loose too. There was also a plastic shield that was available for this.

Thanks, that was very helpful. I also had an issue with coolant leaking. The coolant level was low because of this problem, the van over heated causing the coolant to boil & spill over. Some coolant got on the serpentine belt & caused it to slip off. Why does the belt slip off? Could this be related to the answer in the last reply or is it something totally different?

Coolant won't cause the belt to slide off on its own but it has a lubricant in it that will prevent the smooth back side of the belt from sticking to the pulleys. Along with that, you'll need to sight from the top down and look at all of the pulleys to check for belt misalignment. That's harder to do than to say. When you look straight down from the top you should only see the belt around the highest pulleys. You should not see 1/8" or so of the belt peeking out on a lower pulley. If it is, that usually sets up a squeal, but not always. When you can see a little of the side of the belt that's because the pulley or one to either side is tipped.

A less common cause is a broken harmonic balancer. The cast iron outer ring is glued to the hub, and when that glue lets go the ring can walk out away from the engine. Proof of that is you'll see the belt is running around the smooth pulleys in a place different than where the paint was previously worn off.