We can use light waves also to determine the depth of ocean bed. But they cannot travel faster in water. Sonar uses sound waves and sound waves travels faster in water(fluids) than in air(fluid with less viscosity compared to water). This is because liquid molecules are much comparatively much closer than air molecules. As a result, the energy is quickly transferred from one water molecule to other.
In sonar, the time taken by the sound wave to the ocean and back is calculated and we already know the speed of sound waves in water. By knowing these two quantities, one can simply use Distance = speedxtime to find the depth of the ocean.
