waterline

Concise Oxford English Dictionary © 2008 Oxford University Press:
waterline/ˈwɔːtəlʌɪn/

▶noun

• 1 the level normally reached by the water on the side of a ship.

• 2 a line on a shore, riverbank, etc. marking the level reached by the sea or a river.

• 3 a linear watermark in paper.