freeboard

noun

1. The vertical distance between the waterline and the uppermost watertight deck of a vessel, or the vertical distance from the waterline up to the top of the gunwale (of an open boat).
2. The distance between a water level and the top of something that contains or restrains it (such as a dam).
3. The distance between the top of sea ice and the water level.
4. A type of skateboard which simulates the movement of a snowboard when used on a downhill coarse, allowing snowboarding techniques, which has an addition of two centerline casters that extend below the traditional skateboard wheels and bogies.