I believe that adding water helps to keep the meat moist, while others argue that it dilutes the flavor of the roast.  If you like your pork roast to be very moist, then you may need to add a small amount of water to the bottom of the pan.  However, if you prefer a more well-done roast, you may want to cook it without water. Although, Added water during cook time help to prevent burning on the bottom of the pan. Ultimately, the best way to decide is to experiment and see what works best for you.