I do not claim any sort of complete knowledge of these marinas, but am aware of a few. There are two in/near St Mary's GA that Im aware of and a couple in Fernandina beach, both accessed from St Mary's inlet. There are also a couple in Daytona Beach, accessed through Ponce De Leon Inlet. Been Through it, but not my favorite). Through that Same inlet there is one just before the Fixed bridge in New Symrna Beach and a small one near the inlet. The former is inside a bascule bridge but I don't think there are any height restrictions until you get to the fixed bridge. Near Cocoa Beach there are two marinas and a yacht club in Port Canaveral with no height restrictions. After going through the Bascule Bridge and the locks at Port Canaveral there is the Harbortown marina on the barge canal
with no height restrictions. There are several marinas in Charleston itself with no height restrictions. There might be some Marina's in the Jacksonville
area that have no height restrictions, at least for 72 feet. I have flown over Hilton Head
Island and saw a marina there that appeared to be outside of any bridges. You might try looking at the Brunswick Ga area as well. That is a major port and I believe the bridge there is on the order of 100ft+, though I've never actually gone in there. The only gotcha for that area is paper mill smell.
bob marinas along the Atlantic ICW is pretty good for listing them, the trick is knowing which ones are outside of the fixed bridges. Activecaptain is a pretty good source of knowledge as well.