|I suppose I should explain what the sea is. The sea is a body of salt water that covers 70% of our world's surface.|
|— Grisha Jaeger defining the sea after accounting Eren Kruger's slaughter of PS officials|
The sea covers around seventy percent of the planet's surface. Unlike the freshwater lakes of Paradis Island, the sea is made up of saltwater. It contains such great quantities of salt that no merchant could ever deplete all of it in a lifetime.
In the years before the Great Titan War, the First Reiss King of the Fritz family crossed the sea from the continental mainland over to Paradis Island. In this land, he relocated the capital of Eldia and abandoned the conflicts of the mainland, leading to the Great Titan War in his absence.
In the years after the Great Titan War, the Public Security of the new nation of Marley would frequently sail across the sea to the "borderline" of Paradis Island, an outpost where undesirable Eldian citizens would be turned into mindless Titans and left to wander the island.
By some means, a man of the Arlelt family came to possess a book which revealed geographical secrets of the world beyond the Walls, including the existence of the sea. Eventually, he gave this book to his grandson Armin Arlelt, and it became his lifelong dream to see the sea. Armin shared the secrets of the book with his friend Eren Jaeger, and together they held onto the hopes that one day they would see the sea and the world beyond the Walls, though Eren eventually stopped talking about this dream out of concern that Armin would join the Scout Regiment.
In the year 851, the Scout Regiment sets out for an expedition beyond Wall Maria for the first time in six years. After finding a Titan crawling across the land, they follow its trail and reach the site where Marleyans turn Eldians into mindless Titans at the edge of the island. At this time, Eren and Armin finally saw the sea as they had dreamed they would for many years.
After Marley's Warriors' return to the continental mainland in the aftermath of the Battle of Shiganshina District, the ocean becomes a major scene of combat between Marley and the Mid-East Allied Forces in the Marley Mid-East War. Marley's navy would over the course of four years gain control over the seas, pushing back the Allies' navy to their final stand at Fort Slava. However, Marley's naval skill shows significant weakness during the war, losing over half of its fleet throughout the four-year conflict.
Simultaneously with the Marley Mid-East war, Marley sends a total of 32 scouting ships to Paradis Island. As all of the ships had disappeared, Zeke Jaeger theorizes with Commander Magath that at least two Titans, including the Attack Titan, had blocked or sunken the ships near the island's coast.