Beaches 3-4 hour max from Savannah Ga

Take the ferry over to Cumberland Island, awesome place. Unless you are just wanting to sit on a beach. Cumberland Island is more for exploring though it has a great beach too. You'll have to look up the ferry times if you do it. There are no services so you'll have to take all your food and drink. And July can be pretty hot and buggy. But it has miles of beautiful beach with damn few people, wild horses, a mansion that belonged to the Carnegies (and burned down), and sometimes you can see nuclear submarines headed to their base.

You will be disappointed in the beaches at Jacksonville

There are plenty of areas before you get to Florida on the Georgia coast that are nicer.