A method for generating an extended underwater plasma. A first laser pulse is fired into a body of water to form an underwater optical filament coinciding with a low-energy plasma. A second laser pulse is fired into the water, targeted at the plasma. The second pulse heats the plasma, causing the formation of an extended superheated plasma volume in the water. The two laser pulses can be simultaneous or can be sequential, with the second pulse following the first pulse by up to the filament plasma lifetime. The extended superheated plasma creates an underwater acoustic pulse, wherein the duration, waveform and directivity of the pulse can be tailored by controlling the shape of the underwater laser-generated plasma.