What is a primary purpose of watertight doors on a ship?

The primary purpose of watertight doors on a ship is to prevent water ingress in case of flooding. These doors are specifically designed to create a barrier that can effectively contain water within a compartment, greatly reducing the risk of flooding spreading throughout the vessel. By isolating compartments, watertight doors help maintain the integrity of the ship's hull and contribute to overall vessel stability and safety. This function is crucial in emergency situations where water intrusion poses a significant risk to the ship's buoyancy and survivability.

The other options, while they may describe features or benefits of ship design, do not capture the main intent of watertight doors. For instance, easy access between compartments can be facilitated by hatches or other doors that are not watertight. Similarly, airflow enhancement is not a requirement of watertight doors, as their design deliberately limits airflow to maintain compartment integrity. Lastly, while escape routes are essential, watertight doors are not primarily intended for this function; they serve to protect against flooding first and foremost.