Hero, I think you have some very distorted impressions of how well a stethescope works. In contact with the human body it is only useful for very limited and low range of sounds. lots of dips and peaks, which doesn't matter because it's primarily for listening to low frequency sounds in the first place it can't pick up any high frequency sounds of any kind over any distance in the body. The human ear isn't even within the same realm of discussion, the internal structure of the inner ear and how outer ear interacts with it which allow it to function are incredibly complex, nothing simple at all about it, even the most sophisticated modern sound devices can barely keep up with the technology in the human ear.
There is a very big difference between a device which can produce some tones under water and an underwater speaker.