A waterline is the line of intersection of a vessels hull and perfectly calm water, at a given condition of draught, load, trim and list. On a lines plan, waterlines will be shown for a vessel of upright orientation (zero trim or list) at evenly spaced draughts, producing horizontal sections.
For the section created at a vessels condition of load, see waterplane
The design waterline is the waterline perscribed by the intended condition of load of a vessel (LWL is a similar measure, the length of the waterplane at condition of load. When at design load, LWL = length of the DWL).
The scantling waterline' is the waterline perscribed by the draught at which a vessels structural scantlings have been designed for. It is often the same as the design waterline, however it may be higher.