140 dB under water is not the same as 140 dB in air. For underwater noise reference level is 1 uPa, in air noise level reference is 20 uPa. 140 dB under water would translate to 114 dB in air. Still impressive, a trained opera singer or a typical home hifi system can achieve somewhere around 105...110 dB, but far from a gunshot.
EDIT: in another article it was mentioned that the actual sound level is 108 dB at 1 m which would translate to 82 dB at 1 m in air. 1 m distance is the standard distance to measure the SPL level of an object, eg a loudspeaker. Far less impressive and very, very far from an actual gunshot that is ~140 dB at 1 m distance. Science reporting at its "best".