That much of florida is within 15 feet of sea level?
Does that mean a 20-foot storm surge would wash out the entire southern tip of the state?
Yes. Most of southern florida is lakes and grasslakes, or swamps.
Does that mean a 20-foot storm surge would wash out the entire southern tip of the state?
Storm surge is a factor of coastal shape and distance from the coast. Hurricane Andrew (cat 5) hit with a 7-10 foot storm surge and inundated the coast where it hit eastern florida. Coasts along the upper rim of the Gulf of Mexico can get worse surge because of how the coast focuses water, but it's also higher ground right from the coast in most of the Florida panhandle (excepting the lowland swamps in the river basin north of Port St. Joe).