Underwater detection apparatus and underwater detection method

An underwater detection apparatus (1, figure 1) includes a transmission and reception circuit (3), an acquisition module (17, 24), a boundary setting module (18, 25) and an image data generating module (16, 23). The transmission and reception circuit drives a transducer to transmit a wave and generates a reception signal based on a reflection wave from an underwater target, whilst the acquisition module acquires a water bottom depth. The boundary setting module sets a boundary B1 passing at a point having a depth substantially equal to the water bottom depth, the boundary making a given angle θB1 with a horizontal direction. The image data generating module generates image data IM2 (see figure 7 for 3D image before boundary processing) that represents a location of the underwater target, the image data including colour information. The colour information is set based at least in part on the location of the underwater target relative to the boundary.