Most of the materials incorporated in the TRANSIT LINE are in fact waterproof. However, in order to call a backpack "waterproof" it would need to be 100% submergible, which these are not. Still, we like to call them "weatherproof" due to the materials' properties and the use of things like the water resistant YKK Zippers. There is an excellent article on this topic on Carryology, going into detail on this topic, which we highly recommend:
We hope this answers your question to your satisfaction. Otherwise, feel free to contact us directly at firstname.lastname@example.org to receive a more elaborate answer.