It's possible that the water pump bearing is going bad causing excess strain and belt slipping. If your belt is adjusted correctly and it still squeals, I would check the water pump more closely. All the water is doing is lubricating the pulley/belt interface.
A bushing type bearing can squeal or rattle if it has excess clearance causing the shaft to bounce back and forth inside the bearing bore. That's usually intermittent until the shaft centers itself and stops bouncing.
You may be looking at replacing your water pump if it has too much turning resistance. Maybe a visit to the parts store to compare with a new/rebuilt water pump. They should let you check one prior to purchase, but you may ultimately need to replace it.
