The ocean covers around seventy percent of the planet's surface. Unlike the freshwater lakes of Paradis Island, the ocean is made up of saltwater. It contains such great quantities of salt that no merchant could ever deplete all of it in a lifetime. There are also many species of fish and other aquatic lifeforms living in the ocean that cannot be found within the Walls.
In the years before the Great Titan War, the 145th King of the Fritz family crossed the ocean 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. Over the course of the war, citizens of Eldia had to cross the sea in order to reach the new territory of Paradis Island, but only the wealthy could afford the journey. Only half of the ships which crossed the sea arrived safely at Paradis Island.
In the years after the Great Titan War, the Public Security Authorities of the new nation of Marley would frequently sail across the ocean 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 Arlert 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 Arlert, and it became his lifelong dream to see the ocean. Armin shared the secrets of the book with his friend Eren Yeager, and together they held onto the hopes that one day they would see the ocean and the world beyond the Walls, though Eren eventually stopped talking about this dream out of concern that Armin would join the Survey Corps.
In the year 851, the Survey Corps 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 ocean as they had dreamed they would for seven years.
After the Marley 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 navy ships to Paradis Island, including destroyers and scouting ships. As all of the ships had disappeared, Zeke Yeager correctly theorizes to Commander Theo Magath that the Attack and Colossus Titan had blocked or sunken the ships near the island's coast. Lured over radio by Anti-Marleyan Volunteers, the navy ships are captured by the two Titans to be stored at a newly developed port.
Following the activation of the Rumbling, Eren Yeager leads the Wall Titans across the ocean into the mainland in order to exterminate all of humanity outside Paradis Island. As they reach the coastline, the newly assembled Allied Fleet intercepts them, but the swimming Wall Titans destroy the ships and incinerate the crews. They walk out of the ocean to truly begin the Rumbling, followed closely by Eren's Founding Titan.