Abstract: A sonar device including a transmitter, a transducer, and processing circuitry is provided. The transmitter is configured to transmit a transmission signal. The transducer is configured to project ultrasonic waves in a water body, the ultrasonic waves being generated based on the transmission signal; receive reflected ultrasonic waves from one or more objects in the water body; and generate a reception signal based on the reflected ultrasonic waves. The processing circuitry is configured to generate a plurality of reception beams based on the reception signal; detect a seafloor among the one or more objects based on the plurality of reception beams; determine a limit incidence angle at which the seafloor is detected; extrapolate a shoal line based on the limit incidence angle, the extrapolated shoal line connecting with the detected seafloor; and detect a potential shoal hazard based on a determined depth of the extrapolated shoal line.