When you say wet, I am presuming that you mean just by rain and not submerging.
1st thing is check the alignment of the belt. You can do this by eyeballing along the belt and make sure that it is running absolutely straight.
2nd thing is to have it idling and watch the belt travel. It should be absolutely smooth. If it has any wobble, or jump, see where.
3rd, take off the belt and check all of the pulleys to make sure none of them have any issues. They may have wedged one of them and got it off camber when they did the head. Make sure that all of the pulleys have the same number of grooves.
4th. check the idler pulley to make sure that the bearing is good.
5th. take off belt and install a new Jeep in it.