Submerged gas injection and the subsequent hydrodynamic behavior are phenomena that significantly affect the pool scrubbing efficiency at the injection zone. The Weber number jet transition criteria which have been normally used in the pool scrubbing research could not cover the wide range of the flow rate conditions including high-speed gas jet. In this study, the submerged air injection experiment was conducted at a 5 mm vertical nozzle. Air injection behavior was visually observed with a high-speed camera and analyzed using the shadowgraphy method. The local void fraction along the centerline of the pool was measured by using an optical fiber probe. From the measurement results, a jet transition criteria was suggested based on the nominal Mach number conditions. The effect of flow regime transition on the pool scrubbing efficiency has been confirmed through decontamination factor (DF) experiments. The suggested transition criteria matched with the DF experiment results.