It doesn’t just crawl out and become a new creature. The sea animal evolves through natural selection to survive better closer to the surface. Then natural selection favors the sea animal that spends some time outside of the water. So it’s like a hybrid that can live sea and land. Think of hippos, frogs, and alligators. Then natural selection starts to favor different areas of land and different diets. Some animals lose their efficiency to breathe underwater because it is metabolically inefficient. Those animals have better survival rates and that eliminates the more efficient hybrid breathers. Eventually the ability to breathe underwater is completely lost.