Glossary
Water
One of the two elements of time travel. Water is the barrier between universes: Time Portals and the Fourth Dimensional Construct are made of it.
Water appears whenever the boundary between universes is touched: the bathroom mirror ripples like liquid when Donnie's hand meets Frank's; the flooded school; the water main Donnie breaks with an axe; the cinema screen that liquefies when Frank opens a portal; and the liquid spears that flow from people's chests.
Sparrow's phrase is "the barrier element" - water separates the Primary and Tangent Universes and is also what one passes through to move between them.
Defined in The Philosophy of Time Travel