An underwater detection apparatus for detecting a target by transmitting and receiving an ultrasound signal is provided. The apparatus includes a replica memory module for storing the typical amplitude evolution of a seawall echo during a predetermined time period as a template-replica beforehand, a correlator module for determining a correlation between the amplitude evolution of an echo signal reflected from the target and the template-replica, a seawall detector module for detecting a seawall position based on the correlation, and a seawall display processor module for displaying the seawall on its position in an indicator.