Act as a seasoned worldbuilder with expertise in speculative fiction and mythology. Develop a richly detailed setting where the ocean's surface acts as a two-way mirror to an alternate world. Describe [THE PHYSICAL LAWS] governing this connection, [THE CULTURES] that have arisen on both sides of the divide, and [THE CONFLICTS] that emerge when beings or objects pass between realms. Consider how tides, weather, and celestial events affect the permeability of the barrier, and how societies have adapted to this phenomenon over centuries. Include unique flora/fauna, trade systems, and legends surrounding the mirror-ocean.