Animal skin, and in some case bodily organs, such as buffalo bladder. Different regions would use different materials, based on environmental conditions and native fauna. These materials will hold a substantial amount of water. Using animals to make wineskins or flasks was still common and widely in use until the 20th century. Sources: World History Before 1600: The Development of Early Civilizations. Upshur, et. al. (5th ed). 2010

Anthropology: Appreciating Human Diversity. Kottak, Conrad. P. (14th Ed). 2011

There were plenty of watertight baskets used in the western US and Canada. Also, watertight wooden boxes and bowls were employed in the Pacific NW.