Hydrophone is a microphone which is used to measure sound under water. It uses an echolocation system. You would use a hydrophone if you wanted to examine the sound of fish or maybe if you wanted to listen to find out if there is any sea creature in this part of the ocean. Using several hydrophones make the sound better by making it louder and making it able to reach further distances it is even possible to listen in a specific direction.
The first hydrophone was used by Reginald Fessenden, a Canadian inventor in 1914.