Definition - What does Waterline mean?
A waterline is a level or an interface formed between the water and a body lying in it. In other words, a waterline is the line formed by the topmost layer of water that just touches the body of any object floating on the water.
The term waterline is usually associated with ships, tankers, submarines, tankers, barges and other vessels that float on the water. A waterline is represented by a plimsoll line.
Corrosionpedia explains Waterline
In marine terminology, the waterline is the line where the hull of the ship just touches the water's surface. This water line is called a plimsoll line or the international cargo loading line on transport ships and marks the draft of the ship. It also indicates how much cargo a ship may carry and remain safe.
The waterline level is affected by temperature because warm water is not as dense as cold water and thus provides less buoyancy to the floating object. The same is also true for saline water (seawater) compared to freshwater.
The hull speed is determined by the waterline length. However for sailboats, the waterline length can change as the sailboat heels, and can affect the speed of the sailboat.