wa ‧ ter ‧ line /ˈwɔːtəlaɪn $ ˈwɒːtər-, ˈwɑː-/ BrE AmE noun
1 . the waterline the level that water reaches on the side of a ship etc:
Two torpedoes struck below the waterline.
2 . [singular] the edge of a large area of water, especially the sea, where it joins the land:
As the waterline advanced, the vegetation was swamped.