Freeboard

The vertical distance from the waterline to the deck edge or lowest opening.

This measurement affects dryness, reserve buoyancy and how exposed the vessel is to waves and spray. A vessel with low freeboard may be easier to board but may ship water more readily in rough conditions. Loading, fuel, water, stores and crew weight can reduce it. It is also relevant when coming alongside, setting fenders and judging the height difference between vessel and pontoon.