Sea level rise does not happen overnight: It probably happens over a few hundred years, at the quickest. Plenty of time to modify ports and build new ones. Everybody could walk North out of Florida and beat sea level rise. Water moves North...
Uh...water is gonna move north south east and west. It's not just coming across the southern border. Walls won't stop it.
Plenty of time? And plenty of available land to the north just sitting there waiting to be farmed? We can get all the food we need from Canada?
Your science fiction addiction is once again affecting your judgement about what science can accomplish. We can't even afford to build new roads and bridges yet you've got us mining asteroids and growing wheat in space to feed a starving planet.
I hear there's flooding in the corn belt right now. Trump's trade wars are pushing farmers into bankruptcy. Bees and other pollinators are dying. Rain patterns that crops depend on are changing. There's a lot more involved than just moving crops a little north. Too much rain, too little rain, rain at the wrong time...crops fail, people starve.
What you get is famines, wars, pestilence and wholesale death. Those four horsemen we've been told about.
We can't even manage to rebuild Puerto Rico after a single storm. And a new hurricane and wildfire season are upon us once again...after a devastating winter that has already disrupted farming across the great plains with massive flooding...science better get its ass in gear if it's going to save us.