You are on a ship and you drop a lead line to the seafloor. The line measures 3000 m. How long would it take for sound signal to leave the ship, travel to the seafloor and return back to the ship?

The short answer is that sound travels at approximately 1500 m/s in water. Since it has to travel a total of 6000 m to go to the bottom and back up again, it takes a total of about 4 seconds. (i.e. distance = rate * time; just solve for time).

If you want a more accurate answer, it gets much more complicated because the velocity of sound in the ocean varies significantly as a function of depth (it's affected by temperature, pressure, and salinity). In fact, because of the way those parameters vary in the ocean, sound traveling at a shallow angle actually gets "trapped" at a depth of around 500-1000 m (called the SOFAR channel), corresponding to the region with the slowest velocity. As the sound travels close to the upper or lower limits of the SOFAR channel, it refracts back toward the middle. This is analogous to the way light is trapped within and travels along a fiber-optic cable as a result of total internal reflection. One upshot of the SOFAR channel is that submarines and deep diving whales can communicate with sonar over much longer distances than would otherwise be possible if the sound dissipated toward the sea floor and surface. Another consequence is that the formula for computing the time it would take for your signal to reach the sea floor and echo back is much more complicated than distance = rate * time!