Lets say that you were in a floatplane (whether it be Terry's floating Cub or a big ol' Martin Mars) and the only otherwise-reasonably-suitable landing spot in the waterway where you are trying to go, or the taxi route from your landing site to your destination dock for that matter, is crossed by a movable bridge (aka drawbridge) that will not clear your aircraft when the drawspan is closed. How would you, the floatplane pilot, signal the bridgekeeper to open the bridge so you can land/taxi through the navigable channel provided, especially considering you don't have a foghorn to signal the keeper with, and the keeper only has a marine VHF radio, which you may not have?
Assume that the drawspan can clear your plane when open (always true for bascule and swing spans; landing under a liftspan would be rather challenging though), and that the bridge is operated by a local bridgetender 24/7 (lesser-used drawspans often require prior notice, and there are also remote-control drawbridges out there that use sensors and/or CCTV to indicate the presence of marine traffic to a bridge operator sitting far away).