Watertight doors on ships serve a critical role in maintaining vessel integrity by preventing water ingress during operations and safeguarding against accidents or flooding. The continuous closure of these doors offers significant safety advantages for the ship. 

 Monitoring the status of these watertight doors also contributes to ship security, particularly during non-operational hours. Detecting door openings can trigger alarms in the navigation bridge, enabling prompt responses to potential security breaches. This surveillance is also instrumental in monitoring crew movement during rest periods, reducing the risk of accidents. 

There are three primary types of vessel doors: 

Type A - Doors that may remain open at all times, except in emergencies. 

Type B - Doors that should remain closed at all times, except when personnel are actively working in an adjacent compartment. 

Type C - Doors that should remain closed at all times, except when personnel are passing through the door compartment