wa·ter·line 
/ˈwɑ:tɚˌlaɪn/ noun
plural wa·ter·lines /ˈwɑ:tɚˌlaɪn/ noun 1 the waterline : the level that water reaches on the side of a ship
▪ We repaired the hull below the waterline. 2 [count] US : a horizontal mark on a wall or other surface that was made by water during a flood and that shows how high the water was
▪ We repaired the hull below the waterline.
