We have some major ferry terminals here.. and the noise that transmits into the boat from a ferry running hard against its ramp while unloading can be impressive..
Quite possible you're hearing traffic....
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"