Belt keeps falling off

I just recently put a brand new belt on, so i know that its not worn out. The car runs fine on dry days, but when it rains i have to be extremely careful. If i hit a puddle the belt slips off. I dont know if the tensioner is the cause or if maybe the water got pulled under the belt causing a "hydroplane" type thing against the pully. When i put the belt back on the tensioner feels tight, but i guess it could be tighter. need some help here, i'm tired of getting wet

I'd say it almost has to be the tensioner.

I'd say it almost has to be the tensioner.
x2.

x2.

X3 i cant really think of anything else that would cause that so im 99% sure its the belt tensioner