Final answer:
Single-parent Sailors are required to keep a current passport to ensure their availability for worldwide assignment.
Step-by-step explanation:
The document that single-parent Sailors are required to keep current to ensure their availability for worldwide assignment is their passport. A passport is an official government document that allows an individual to travel internationally. It is essential for single-parent Sailors to have a valid passport in order to be readily available to travel for their assignments around the world.