What does a watertight door prevent?

A watertight door is specifically designed to prevent the passage of water and air between compartments on a vessel. This is crucial in maritime operations because maintaining the integrity of compartments helps control flooding and manage buoyancy. When a watertight door is securely closed, it ensures that any water that enters one compartment does not spread to adjoining spaces, which is essential for damage control and maintaining the overall stability of the ship.

By blocking the movement of water, these doors play a vital role in preserving the vessel's structural integrity during an emergency, such as a hull breach. Additionally, preventing air movement can help control fire and smoke spread between compartments, creating safer conditions for personnel.

The other options do touch on related concepts but do not capture the primary function of watertight doors. For example, while these doors do restrict personnel movement during flooding situations, their main purpose is not to prevent people from moving; rather, it is about sealing off areas from water and air intrusion.