Even if you could slide the serpentine belt off one of the pulleys, you would need to be able to rotate the tensioner in order to get the belt back on. You can buy the tensioners, probably for $40 or so. You can unscrew the tensioner using the bolt with the 13mm head. You will likely have to raise the engine an inch or two in order to get the bolt out. You do this by supporting the oil pan with a jack (and block of wood between jack and oil pan), then unbolting the motor mount on that side of the engine.
And to change the water pump, you would need to remove the timing belt cover, which will involve taking off the motor mount "isolator", as well as the crankshaft pulley.
If course, you may have already done all of this. Sorry I didnt see this sooner.