Well technically, sound (in the air) can be loud enough to kill humans. So I certainly wouldnt doubt that underwater sounds can kill fish. The reason we usually arent so concerned about underwater sounds is that we cant hear them. The sound waves travel far to fast for us to pick up and process.
Now if you want to get into specifics, here goes. Because of the way sound travels underwater vs. in the air, you need to add 62 dB to any air measurment to get its dB underwater. Meaning, 120Db music playing would be 182 Db underwater. Now, from what I can find online, the sounds dolphins make to stun their prey are somewhere in the range of ~200 Db.