A waterline is the line on a ship or boat that indicates the level of the water when it is floating. This line helps determine how much of the vessel is submerged and is crucial for ensuring stability and safety while at sea. The waterline can change based on the weight of the cargo and the amount of water in the hull. In addition to ships, the term "waterline" can also refer to the level of water in a river, lake, or reservoir. Understanding the waterline is important for activities like fishing, boating, and flood management, as it helps assess water levels and potential hazards.