This is a question that maybe the military community could shed some light on:
Several times while surfing/bodysurfing near Dog Beach I notice rapid percussion in the water. If you are sitting on your surfboard you can feel it like a tapping on the bottom of your board, or if swimming, they can be felt inside your body. Put your head underwater and you can hear it, like distant machine gun fire or jack hammering. Short bursts of 5-10 pops. It doesn't seem like the noise would transfer well from the air and into the water, especially when there is no noticeable sound above the water.
Perhaps some cool underwater machine gun training?
Comment (keep it clean & on topic)