A non-transferred atmospheric-pressure argon arc jet is ejected into water from an anode nozzle and examined by optical emission spectroscopy measurement to study thermohydraulic characteristics. The plasma temperature dropped sharply at 10–13 mm downstream measured from the nozzle. Since the arc diameter was found to be almost constant, the arc velocity can be traced from the mass-flux conservation. The velocity dropped sharply at the aforementioned region, where its velocity approximately agreed with the sonic velocity in the water. It is considered that energy dissipation occurred near the sonic velocity of water.