I lived in southeast Georgia for 25+ years (retired from working for the City of Savannah). In all the time I worked and lived there (and during the ten years I have lived in Texas) they never seem to have dealt adequately with street flooding no matter how much $$ they spend on drainage projects. Besides that, just a run of the mill high tide will flood Hwy 80 and make it impassable (the one road leading to Tybee Island)
Savannah is pretty low-lying and the streets do not have any drainage. So yeah, if they get a few inches of rain in a short amount of time, access to Tybee will be impossible. I assume the residents are making plans to stay where they are are for a few days.
Hubby has relatives in Brunswick, Some who live right on the marshes a couple of miles from the access road to Jekyll. I believe they are battening down the hatches as best they can and heading northwest to ride it out.