Sadly, water is a two-edged sword. Too little and the fire still burns and too much and you destroy historical artifacts with water. Notre Dame de Paris houses (housed?) the largest organ in France. It is a Cavaillé-Coll with 5 manuals and around 8,000 pipes. Water will ruin it. It may yet be ruined if it rains with no roof over it. The cathedral has various holy relics, statues, paintings, carvings etc. Many of those can easily be irreparably damaged by water. So, you are damned if you do and damned if you don't. Not an easy decision.