What is a fathom used to measure?

A fathom is a unit of measurement primarily used to indicate the depth of water. This measurement is especially relevant in marine contexts, such as navigation and fisheries. One fathom is equivalent to six feet. Mariners use fathoms to ensure their vessels can safely navigate through water, avoiding shallow areas that may pose a risk of grounding. This unit has historical significance in maritime practices, providing a standardized way for sailors to communicate water depth effectively.

In contrast, the other choices do not align with the primary use of fathoms. The width of a ship, distance in the air, or length of a crew member are measured using different units suited to those specific dimensions, such as feet, meters, or other standard units of length.