The Great Eastern is a legendary ship, added to the game with Game Update 8.
It is a late-game trade ship which has the most cargo slots of any ship in the game, giving it more capacity than the Cargo Ship and Extravaganza Steamer. Furthermore, it has the highest base health of any of the trade ships, being second to only the hulking Pyrphorian Battle Cruiser. For a steam ship, its movement speed is high, being surpassed only by the Pirate Monitor. It has 3 item slots, on-par with the Extravaganza Steamer and one less than both variants of Battle Cruisers, which, alongside the 8 cargo slots, makes it a fantastic choice for Expeditions.
The Great Eastern is first offered by Sir Archibald Blake after building the World's Fair via a quest called The Floating City. Players may only have one Great Eastern at a time unless they have the Land of Lions DLC and have constructed a Research Institute. Permits may be researched allowing the construction of an additional Great Eastern. The permit is one-time-use and must be researched again if you wish to construct another one.
After losing or selling their Great Eastern, the player may purchase a new one for 750000 at Sir Archibald Blake's Harbor. If you have the Land of Lions DLC, then you must also have a permit.
SS Great Eastern was the last of three great steamships built by the legendary railroad engineer Isambard Kingdom Brunel. Launched in 1858, she was by a huge margin the largest ship ever constructed according to any metric. She would hold this record until the very end of the 1800s, and her last record--passenger capacity--was not surpassed until 1913.
While an engineering marvel, Great Eastern was an ill-starred ship. She bankrupted the company building her, and it took four attempts to launch her. She was too large for passenger travel to Asia, the role she was designed for. She was too slow and inefficient for trans-Atlantic travel compared to smaller ships, and was never profitable. As a result, she was out of passenger service by 1864, and spent the rest of her life in odd side roles--the most notable being her key performance in laying the first successful Trans-Atlantic Telegram Cable.
Great Eastern was broken up in 1888-1889, and very little remains of the legendary ship today. Yet in her life she was unchallenged in scale, and it would take until ten years after she was gone for any ships to come close. In an era of legendary engineering feats, she stands out. 
While the Great Eastern was only added to the game in Game Update 8, traces of it were already seen in the files in Beta. This early version of the ship used the same model and textures as the Extravaganza Steamer, was contructed with a Specialist Item and even had offensive statistics, suggesting it was armed (which the real-life Great Eastern had never been). It featured further enormous statistics such as 6 item slots, 15.0kn speed and an upkeep of a whopping -5000 Credits.
Great Eastern is the ship seen on the Anno 1800 main menu, though with a nameplate on her bow that says "Dahlia".
Great Eastern features both paddlewheels and a single screw propeller; twin propellers were an experimental concept when she was constructed.