I thought about that, too. (By the way, if you stand in the surf on the beach, in the waves, watch as the retreating water washes around your feet. It sculpts the beach sand into a shape that looks like a carolina bay.)
In order for stranded icebergs to have been the cause of bay formation, there would have to have been many icebergs off the coast of Georgia and South Carolina. Those areas had warm currents (the Gulf Stream) even then, so icebergs would have been highly improbable. Also, during the glacial maximum, sea level was as much as 330 feet lower than it is today. A tsunami would have had to be large enough to raise the coastal water level by around 600 feet in order to reach the upper coastal plain...and it would have been along a 1000-mile stretch of coastline...I don't think there's been a tsunami like that at any time during the tertiary period.
Well, I take that back. The meteorite that hit off the coast of Virginia at the end of the Eocene might have been able to cause one that large. However, the climate was warmer then....not glacial....and sea level was much higher than it is now.