Air horn - the PFD whistle's louder bigger brother. I don't carry mine on me, but I keep it in the day hatch within easy reach. When I'm in a channel with poor visibility I take it out and keep it in my lap.
Pro tip - point it *toward* the boat not paying attention, or at least away from your ear.
It's an attention getter and because it sounds *big* it makes people look around quickly!
