The estimated depth of the ocean at any place can be measured with the help of:

A device known as SONAR measures the depth of an ocean by sending ultrasonic waves toward the bottom of the ocean. 

A sound wave with a specific frequency is allowed to transmit into the water. This sound wave will travel through the water and reach the sea bed. Once it touches the sea bed, it will reflect the surface. Ships that have echo sounders have a device called transducers which will transmit and receive this sound wave. Then the time taken by the wave will be recorded. The velocity of the sound wave is noted and we can know the depth of the sea. 

Hence, the depth of the ocean at any place can be measured (estimated) with the help of ultrasonic waves.