A water turbine of the present invention comprises: a rotary shaft having buoyancy; and a blade formed in a spiral shape so as to have pitches at predetermined intervals along the outer peripheral surface of the rotary shaft. Therefore, a low-speed flow of a tidal current is converted into a fast rotational movement by the water turbine comprising the spiral blade, so as to maximize the rotational efficiency of the water turbine, thereby enabling power generation efficiency to be improved.