the level that the water reaches along the side of a ship （船的）水线，吃水线 The ship was struck below the waterline by a single torpedo. 该船被一条鱼雷击中了水线以下。 - (figurative)
If the vote goes against her, the prime minister may well be holed below the waterline. 如果投票反对她，首相很可能会陷入困境。