What is the term for the speed of a vessel through water?

The speed of a vessel through water refers to the actual speed at which the vessel is moving relative to the body of water it is traversing. This concept is often referred to as "speed through water." It is critical in navigation, particularly for determining how currents, tides, and other environmental factors can affect a vessel's actual path and maneuverability.

In the context of the options provided, "speed over ground" would be the term used to describe the actual speed of the vessel relative to the seabed or land beneath it, factoring in any movement caused by currents. "Velocity" is a broader term that includes both speed and direction but is not specifically focused on marine navigation. "Water speed index" is not a commonly used term in navigation contexts.

Thus, the accurate understanding of the term that specifically denotes the vessel's speed through the body of water itself aligns with the provided choice that clearly articulates this concept within the context of maritime navigation.