I had two jetskis start to squeeze between my boat and another boat I was rafting up to in a crowded anchorage. Both drivers were looking backwards at who-knows-what, so I used my "too big for my boat" horn. I doubt they knew the meaning of 5 short, but at least they turned around before anything bad happened.
Friend of mine has a 100' house boat on a large lake in the Heartland. Has a train horn on it. Once in a while he'll pick a time, and send out a mass mailing to his friends to listen. The record is a little over 5 miles.