The present disclosure provides an underwater detector including a power source for applying a predetermined voltage, a voltage control circuit for controlling the voltage based on a control signal, a gate signal generating module for outputting a gate signal, a switching circuit for outputting a reference signal based on the voltage controlled by the voltage control circuit and the gate signal outputted from the gate signal generating module, and a transducer for transmitting an ultrasonic signal underwater, an envelope of which being controlled based on a waveform of the control signal by being applied with the reference signal.