Had an air horn once, but the spout filled up with water and the thin metal diaphragm corroded away. Air horns for marine use are better made than auto products and usually have stainless steel innards.
Yes, I don't know why BMW who could be champions of bike safety, fit skimpy horns and a control so illogical in positioning, mirrors without a blind spot curve and a rear tail light that is pretty dim. All these simple things to do, whereas cage owners are spoilt for choice with multiple air bags, side impact crash protection, impact resistant fenders, decent lights including bright led for braking, ABS and seat belts.