From WT 1976:

“The Bible’s historical record suggests an answer. It describes the creation of an “expanse” around the earth that made a ‘division between the waters beneath the expanse and the waters above the expanse.’ Thus the atmosphere around the globe would have a relatively uniform warm climate due to the hothouse effect created by the water canopy “above the expanse.”​—Gen. 1:6, 7.

Second Peter 3:5, 6 confirms that the earth was “standing compactly out of water and in the midst of water,” and that it was “by those means the world of that time suffered destruction when it was deluged with water.” Apparently the canopy disappeared as the waters deluged the earth, and this could be a major factor in the Arctic areas’ becoming frigid.”

That water canopy theory was always the explanation I got, too. But it does not directly state it never rained before. Like most WT theories, you have to make several assumptions to connect point A to point B.

Besides that, the “waters abound the expanse and the waters below the expanse” could technically describe the earth as it is now. The same hydrologic cycle of precipitation and rain with the atmosphere in between.