I have used a boat air horn. The Air Zound for bicycles is louder. The boat air horn does not work well in the cold. Neither does the Air zound. The boat horn does not work well when pointed into the wind or anywhere near forward if you are going pretty good. You can't tip the air horn much as it needs to be close to upright.
It's not any better than yelling. It confuses some people and they look for a car. They may not realize it is you.