Take a closer look at the pulley on the water pump.
The front half of the water pump pulley is on threads, and screws away from the rear half of the pulley to loosen the belt. There should be a nut and a set screw on the side of the front pulley half. Loosen that, then turn the front half of the pulley while holding the fan, or the back half of the pulley.
If it's been a while, it may need some persuasion with penetrating oil. You don't want to use heat, or you'll burn up the seals on the water pump.