The modern tools includes several forms of sonar(detection by sound reflection).
sonar
Sonar
If you are meaning to U-boats as in World War two? Than U-boats certainly mean "Underwater Boats"
by flying underwater
Submarine
If you are meaning to U-boats as in World War two? Than U-boats certainly mean "Underwater Boats"
Those objects are called "buoys." Buoys are typically floating markers that help guide ships, boats, and other watercraft by marking navigational hazards, anchorage points, or underwater obstacles.
Objects that are designed for movement through a fluid medium, such as airplanes, cars, boats, and even underwater robots, need to be streamlined. Streamlining reduces drag, making it easier for these objects to move efficiently and swiftly through the air or water.
United States.
Most likely Germany, they had the U-boats (underwater boats)
Sonar is the technique of listening to sounds underwater to detrmine what is there. In passive sonar, objects not generating any sound will not be detected. Active sonar means that sound waves are generated and emitted. The device then listens for the sound being reflected back. That detects objects such as obstructions/rocks as well as other boats or underwater objects. The reflected wave is measured and the time delay noted as well as the direction of the reflected sound wave. These give the user a very good idea of the type, size as well as speed of the object.
the chinese people