Operating a floatplane/seaplane on water through a drawspan?
When I sail we use simple air horns to signal the local drawbridges. Something like this one is usually kept on the boat. I see no reason not to keep one in your float plane if you anticipate need to hail a draw bridge or just for general marine hailing. If memory serves, it's one long blast (4-6 seconds) followed by a quick one. Similarly you could opt to carry a hand held marine radio in your plane. 

Now all of that is assuming you knew you were going to encounter a bridge. If you did not plan for it then you have a few options. If memory serves some (if not all) of the bridges have land lines and if you have a cell phone you may be able to call them (provided you can find the number). If you are flying a float plane you may (should?) have a PFD on board. Many PFDs have whistles attached. This, in theory should suffice to hail the bridge. The bridge attendant may see you floating there and open the bridge as well. 

You can always try, um, yelling at the guy... 

Now what if the bridge is broken, there is a long train stuck on it and the attendant stepped out for a cigar break and has no intention of returning for a while? You are in the end of the day in a float plane and you have a unique ability that most ships don't have, you can go over the bridge...