Tissue breaks apart in water, but not if thrown off a tall building. This is mainly due to the tissue's molecular structure and the high amount of air resistance.
Tissue is made up of mostly cellulose fibers. In dry paper, the cohesive force between the fibers is strong. However, once water is introduced, these bonds become much weaker. This is because the cohesive forces that bind the cellulose fibers are hydrostatic in nature. And since tissue is hydrophillic, more water enters the bonds and more of the structure is weakened.
Tissue does not break when thrown off a tall building because it has a large surface area to weight ratio, which displaces air relative to its movement and slows it down.