I’m assuming water breaks down the cellulose or dissolves something in the paper, in that case are there other liquids that you could soak paper in that wouldn’t weaken it?
It’s actually more like the water acting as a lubricant by expanding the space between the fibers in the paper. This lets the fibers slide past each other more easily. Normally the friction between the tightly packed fibers is what keeps the paper together.
Paper is made from cellulose fibers that have been pressed together and allowed to dry. Thanks to the pulping process, these fibers are relatively short and arranged in random directions. When the paper gets sufficiently wet, the cellulose fibers loosen up from one another and the paper dissolves.