For 20 years we used a handheld car horn from the junkyard with a switch mounted on it on my friends boat. Had to open bridges to get from fresh water
. Got my own boat, did the same thing, but had to open another bridge. The bridge didn't open, when I called on the radio
, the bridge operator said to me, No I can't hear your car horn.
I intend to buy an expensive chrome plated marine
horn that looks identical to the junkyard horn and try that on the bridge someday.
Disclaimer, just as for lights, sound signals have rules as to distance to be heard and frequency of the sound that they have to meet. Don't know if the car horn meets specs, but it sure sounds like the one from the marine