Because of the standardized technique used some believe that the Nemi ships were actually standard grain ships minus their anti-ship worm lead armor with purpose built superstructures plopped on the deck.
That possible. Also from Caligula/Gaius' reign, the great ship built to transport an obelisk was a spectacle in its own right, remaining a tourist attraction at dock until it was reused by Claudius as the form for the pouring ot the concrete when C built the Portus all-season port for Rome. It's not out of the question that at least parts of that great ship have survived, buried end down, in the now-landlocked port. The internal structure, which would be interesting to examine, may very well still exist as a negative impression in the concrete.