In large bodies of water, do you not observe oscillating sounds? I can't explain it very well (I forget the official name) but it's like pushing a wave onto another wave at just the right frequency so it creates a huge wave, could the same thing not happen with sounds in water? It might explain the ones that build up and then gradually decrease. I don't know much about this and am probably wrong but it's something I've always wondered.

If you had enough random sounds going off in the water, given enough time some would group up together I imagine to create freak sound waves.