"Ham", you'll be back from your trip soon and I see no one has answered your last question. That's probably because it is off topic and the right person isn't reading this thread. Your question impinges on areas of physics and biology which I'm not fully qualified to answer but I have a general idea.
We all know that sound travels faster underwater but the physics of sound generation and reception are also quite different. The subsurface is a strange world where subjective perception of volume, tone and direction is like a hall of circus mirrors. This is because our ears only work as designed when the eardrum is suspended between two air bubbles. The ear is designed to receive, amplify and convert vibrations generated in air. Specifically, the ear drum reacts to tiny pulses of air pressure associated with vibrations which we interpret as "sound". Moreover, the brain is designed to discriminate sound which hits each ear at slightly different times and at a speed of about 1100 fps. The higher speed in water exceeds the brain's ability to differentiate, to identify direction, but we are getting ahead of ourselves.
Sound is transmitted efficiently in liquid and solid media. However, the reception of sound frequency and amplitude depends on the compatibility of the receiver and transmit medium; this is called impedance. We know that the impedance of the human ear(ear drum) is high relative to air. This permits sensitive response to vibrations and good acuity. In fact, the human soft tissues have a sound frequency impedance very close to water. As we know, that's good for hearing above water, but underwater this is confusing and inefficient to the ear because the underwater sound waves are thus transmitted completely through the body and not specifically through the outer ear canal. With little being received through the ear canal, it's a wonder that we can hear at all since, under this theory, the amplification provided by the ossicles(ear bones) is mostly lost. However, some sound is being received as we all know. Instead of the normal route the molecular vibrations are probably transmitted directly to the cochlea without much corresponding vibration of the drum and ossicles.
Overall, the human body acts like an antenna where the skeleton has specific "tuning" which can vibrate and conduct sound at certain frequencies depending on the length, size and density of the bony structures. These sounds are conducted through the skull to the inner ear and auditory nerves much like the act of striking a wet railway track with a hammer. The long bones and spine are fairly well isolated from the skull by cartilage which probably doesn't vibrate efficiently. Hence, one might assume that the skull and to some extent, the small ear bones, are the primary elements for conducting sound. Viewed as a pure antenna, with its small size relative to sound waves, the skull is probably most efficient at receiving short waves associated with high frequency sound. That doesn't mean that we can't hear some middle and low freqs, evidently we can, just not efficiently.
Underwater, the audible rotation of a high speed propellor is just one component of the general racket these things make, yet all we hear is the high pitched whine. Same with the SCUBA regulator. Release of pressurized air through the regulator valving produces ultra sound and high frequency sound. Release of low pressure bubbles produces low frequencies. These frequency differences suggest why you can hear only part of the regulator's cycle from a long distance. You hear your buddy's high frequency inhalation but cannot hear his low freq bubbles. At any distance beyond a few inches we are hearing short wavelengths(high freq) more audibly; meanwhile, the fish are hearing more long waves(low freq), eg the bubbles and, apparently, quite clearly.
I've done some dissections on fish which were speared for the table. The drum family have distinctly unique bony structures obviously designed for underwater sound. They probably have inner ears adapted to low frequency detection. Based on what I've seen underwater, and I guarantee you that the underwater hunter is very aware of these things, these fish can hear booms, grunts, burps, bubbles, and anything associated with low frequencies better than we can. They seem oblivious to high frequencies although that doesn't necessarily mean that they can't hear them. Their behavior is probably a function of the fishes' evolution including its own ability to communicate. I believe also that I have seen these observations reported in scientific literature but I can't supply references. As I said, this is not my field but I believe my rather broad inferences are close to the mark.
PS: Further inquiry may lead you to physiological features which are unique to the fish such as the lateral line. This is an ultra low frequency(infrasound) "hearing" device which may contribute to the fishes sensitivity to bubbles. That issue is not as clear to me, however.
