What device is used to measure the speed of a vessel through water?

The device used to measure the speed of a vessel through water is the Pitot tube. This instrument works by measuring the difference between static and dynamic pressure created by the movement of water past the vessel. As water flows into the opening of the Pitot tube, it exerts pressure that can be translated into a speed reading. This reading provides essential information for navigational purposes and helps in operating a vessel efficiently.

In contrast, a depth sounder is primarily used for measuring the depth of water beneath a vessel and does not provide speed information. A sextant is an instrument used in celestial navigation to determine the angle between celestial bodies and the horizon, which assists in navigation but also does not measure speed. A manometer is a device that measures pressure, typically used in various scientific and industrial applications, rather than specifically in marine settings for measuring vessel speed. Each of these instruments serves a distinct purpose, making the Pitot tube the correct choice for measuring the speed of a vessel through water.