Chris - It sounds like your water pump pulley may be for a slightly wider belt (1/2" or even5/8" ???) than the alternator and crank pulley. The belt should fit similarly in that water pump groove as it does in the other 2.
Would be super helpful for everyone if you could post pictures of the fit of belt in each pulley. Going back to the lists I posted above, for every different width belt, there are corresponding different width pulleys. It's very easy to get a mismatch.
