Throwing pulley/alternator belt

1. Get it propperly aligned.
2. Get it propperly tensioned
3. Get correct belt for the pulleys.

My bet is if you setup a go pro camera you would see the belt whipping around and with it not aligned correctly walking up the side of one of the pulleys.

Your photos look like you have a 1/2" wide belt in 3/8" wide pulley.

Also look at the pulleys for bent areas

Also if you are using an aluminum water pump with brackets for a cast iron one or visa versa, that induces an offset issue.