In a large mixing bowl, combine the water, yeast, flour and fine sea salt. You can mix initially with a wooden spoon or danish dough whisk, but will need to finish up by hand. Pinch dough together until no dry bits remain. Mixture will be thick and sticky.
In a separate large bowl, drizzle 2-3 Tablespoons olive oil to coat the bottom. Transfer the dough to the oiled container. Turn the dough so that all sides are coated in olive oil. Cover with plastic wrap or lid. Let rest at room temperature for 30 minutes. Refrigerate for 12-48 hours. Dough will rise to double in size.
Remove from refrigerator and allow to rest for 30 minutes in the container, slightly warming up.
Prepare a half sheet baking pan (18x13-inches) by coating the bottom and sides with 2-3 Tablespoons olive oil. Turn the dough out from the container and onto the oiled pan and flip so that the smooth side is facing up.
Gently stretch the dough to fill the pan. Use the backs of your hands and knuckles to stretch the dough without tearing it. If the dough is springing back and not staying in place, let it rest 10 minutes and come back to it. Cover the dough lightly with plastic wrap or a dome plastic pan lid. Let rise until it reaches the level of the pan, about 1 hour. Towards the end of the final rise time, preheat the oven to 450°F/230°C.
Uncover and lightly oil your fingertips. Gently dimple the dough by pressing into it with your fingertips. Press all the way to the pan. Create a moderate amount of dimples throughout the dough without depressing too many of the air bubbles. Sprinkle 1-2 Tablespoons of sea salt flakes on top.
Bake 18-24 minutes or until golden brown.
Brush the top of the focaccia with 1-2 Tablespoons olive oil.
Remove focaccia from the pan with a large spatula and slide onto a wire cooling rack. Allow to cool at least 10 minutes before slicing and serving.
Store any leftover focaccia in an airtight container. Best served the day of, but will remain fresh for up to 2 more days.