why does the sepentine belt come off when it is exposed to water...ex: rain,puddles?

Often when this happens there is a worn bearing on one of the pulleys that is causing a slight misalignment, often not even noticeable, if the tensioner is new, i would be looking carefully at all the bearings that carry the belt pulleys and also a close look at the harmonic balance as well, may sure that there is no wobble when it runs.

Very common problem that can cause this is the belt tensioner. Look to see if the belt is in perfect alignment with all pillows. The tensioner usually gets bent off alignment and needs replacing.

I had the same problem on my 1996 Grand Voyager. I changed the belt and that took care of it. The simple fix it the best unlees your the repairman then do all the rest first. The belt was replaced 4 years ago and it hasn't jumped off again so now I can play in the puddles again. good luck