Design a futuristic seaport building inspired by the organic shape of a sea shell, engineered to float gracefully on the surface of the ocean. The structure should be sleek and aerodynamic, harmonizing with its surroundings while offering stunning views of the vast, open water. Below the floating building, reveal a mesmerizing underwater city of Atlantis, alive with neon-lit skyscrapers, bustling streets, and advanced technology. On the surface, luxury yachts gracefully depart and arrive, their elegant lines reflected in the shimmering waters. The overall design should seamlessly blend nature, technology, and luxury, capturing both the tranquility of the sea and the dynamic energy of a thriving maritime hub.