Just anecdotal I don't know for sure, but I've experienced it a couple of times, at first thinking it was dolphins or whales, but it's more of a continuous noise and seems quite positional as if it is a reflection and superposition... pretty much all boats have some kind of radar that operate in the microwave frequency range, so it seems like a strong candidate for the source.
It wasn't so strong to be nauseating, just noticeable and weird sounding... then again I wasn't trying to sleep through it.