It definitely is a thing; see my reply above. It's the engines. I was able to experimentally connect /one/ source of hum to tug boats specifically. Interestingly, though, the huge container ships that passed by did not make an audible noise.

One unfortunate effect of the name "the hum" is it kind of implies a single source. Realistically, it's probably a whole collection of sources, including some real physical phenomena, and some tinnitus-like audio hallucinations.

Not sure what it is specifically. When living on a sailboat, under the waterline, you can hear it quite distinctly.