Air draft = height above water = الارتفاع فوق سطح الماء/ الارتفاع فوق الماء

"If you opt to go through without a pilot prepare well ahead: make sure any mast or canopy is taken down in plenty of time and that everyone comes in off deck. Be sure to know the air draft (height above water) of your vessel. This is usually to be found printed on a plate next to your boat's steering position. Check this against the gauge board next to the bridge. If there's a strong curent, be sure to be well under way with enough steerage to cruise cleanly under the bridge."

Vessel = a ship or large boat = سفينة


Sources:
http://www.waterscape.com/canals-and-rivers/river-bure/boati...
http://www.bargeholidayfrance.com/boat/
http://excelmathmike.blogspot.com/2010/02/do-you-prefer-cape...
http://www.wordreference.com/definition/vessel
http://www.wordreference.com/enar/vessel