Underwater detection apparatus and underwater detection method

An underwater detection apparatus is provided. The underwater detection apparatus includes a transmission and reception circuit, and processing circuitry. The transmission and reception circuit drives a transmission transducer to transmit a transmission wave and generates a reception signal based on a reflection wave including a reflection of the transmission wave on an underwater target. The processing circuitry acquires a water bottom depth, sets a boundary passing at a point having a depth equal to the water bottom depth, the boundary making a given angle with a horizontal direction, and generates an image data that represents a location of the underwater target based at least in part on the reception signal, the image data including a color information, the color information being set based at least in part on the location of the underwater target relative to the boundary.