The t-belt has to be removed cause it can't get wet, especially by coolant. The water pump sits below the t-stat and once you remove both, LOTS of coolant will spill out onto the t-belt and that's a BIG NO NO...

This is why at the dealership I work at, we sell a T-belt Package:
Consists of water pump, t-belt, tensionor, t-stat, cam seals, crank seal, drive belts and lastly...Nissan coolant.