Experimenting with reverb and frequency on my "meaty gunshot. Wav" sfx resulted in this; hopefully this conveys the audible sense that the gunshot is being fired underwater. Created and edited with audacity.
A single, meaty gunshot created by editing my "single gunshot 4" sfx. I think this one sounds and feels a lot better. Created with audacity. Use however you see fit.
Took a gunshot from shawnyboy (http://www. Freesound. Org/people/shawnyboy/sounds/166191/) and edited it in audacity so that it plays in reverse immediately after the first sound.