Watertight

A watertight ship door or hatch withstands a so-called 'water column' (water pressure) from both sides.

The closure between two compartments below the waterline will typically be watertight. It is essential that these compartments are resistant against great water pressure, as is the case during a major water leakage. Due to a watertight closure, the leakage won't spread and the vessel will stay afloat. Because this watertight closure is so essential for the safety of the vessel, classification is required for in most cases. 

The watertightness of a ship door or hatch is tested by mounting the door or hatch into a tank and applying a certain water pressure from the inside. If the water is coming from the outside of the door or hatch, the door leaf or hatch lid is pressed into the seal by the water pressure which helps to seal. Water coming from the inside, will counterwork. This way, the watertightness of a product can be easily measured.