Matt, I think you're finally 'getting it', thanks to Woody clarifying things better than I can, especially for the diagram. The dynamotor could run off of the battery, but not if it was wet. If the engine could run the generator, but the dynamotor was wet, the radio would still not work. That part of the aircraft would have needed to be not just above water, but dry, to transmit. That pretty well dictates that it needed to be above water all of the time. Dip a dynamotor in salt water and it stops producting high voltage, even after drying out. It would need to be rinsed with fresh water, at a minimum, unless it had arced and made carbon tracks on the insulation. In that case it's permanently inop until rebuilt.