WD,
Here's one, since you want to go really big:
http://www.groverairhorns.com/A guy from my last office had a set of ship horns on his truck. Way loud enough. Loud and a much deeper tone than regular auto air horns.
And these guys make'em for ferries:
http://www.airchime.bc.ca/Not sure how you'll get these things mounted on a bike, (or if you'd really want them so close to your own ears) but I'd love to see it.