It was a tugboat and it did indeed blast those Nathan horns like there was no tomorrow (I think those shrieks must have shattered windows in Berkeley and woke up Emperor Norton in Colma!), at a little after 11 p.m. last night, IIRC. The USS Mazapeta is an operable and well-kept World War II era ex-Navy tugboat that is privately owned and docked in Alameda. It once served at Mare Island.
Glad that you were among those at Pier 39 who were able to hear and see the Mazapeta. From the tug, we could see you folks gathered there and were occasionally blinded by your random camera flashes (Geez, Myrtle, I don't think that $5 disposable point-and-shoot film camera you bought at Fisherman's Wharf is going to capture a moving tugboat 500 feet away in pitch black darkness! Guess you're right, Herman!).

Happy New Year ... Chris Goepel ...