Gameplay The player will take the role of the first Big Daddy ever created in Rapture, who possesses superior speed and intelligence than subsequent Big Daddies, though he lacks some of their strength. He has seemingly survived the events of the first game. As such, the Big Daddy's drill and rivet gun will be weapons that the player may use, but the drill will overheat if used constantly. Alternatively, the player may also use plasmids, and their effects have been changed, while "upgrade trees" will provide unique ways to upgrade plasmids, however other Big Daddies won't have the ability to use them. A new feature added is the ability to dual-wield plasmids and weapons at the same time. Further Big Daddy abilities found in the first game, such as performing a shoulder dash, will also be an option. The player will also be able to step outside Rapture into the ocean due to the diving suit they wear. Here the player can explore the ocean floor, have some down time, marvel at the city from the outside and even pick up the special sea slugs at the ocean floor to gain more ADAM. These sea slugs are gene-modified so they create ADAM. The slugs were the original idea instead of Little Sisters in the first BioShock. The Big Sister also appears to be a very serious threat to the player. When a certain number of Big Daddies are killed she will attack. Unlike the Big Daddies, she is lightning quick and arguably more powerful, able to extract ADAM from her enemies through a harvesting needle, straight into her bloodstream, which fuels "telekinetic abilities beyond anything previously thought possible".
As a "rogue" Big Daddy, players will attempt to capture the Little Sisters from other Big Daddies who roam Rapture. Once caught, the player will again have the option of choosing the fate of the Little Sisters. Harvesting remains a possibility in order to gain more ADAM, but rather than rescuing the Little Sister, players will instead be allowed to adopt them. With this option, Little Sisters will ride around with the player's Big Daddy, and will look for ADAM to harvest from corpses; in a manner similar to the museum area of the first game, the player will then need to ensure that splicers will not harm the Little Sister as the harvesting process takes place. The Splicers will not only damage the Little Sister, but will also delay the process of gathering the ADAM. Rescuing, while not initially an option for the player, will become available at some point in the game. Additionally the Little Sisters will also warn the player when the Big Sister is approaching.
Returning features from the first game include the ability for the player to regenerate through the use of Vita-Chambers, and audio diaries. In addition to the single-player story, multiplayer will be a feature in BioShock 2, but this will not take the form of co-operative play.
It has also been confirmed from the latest gameplay trailer that Gene Banks will return and that previous areas of Rapture still exist, such as the Kashmir Restaraunt where the player started out the first game, complete with the iconic statue of Atlas.
Set ten years after the events of BioShock, a number of towns around the world have been reporting kidnappings of young girls by attackers described as "thin", with red lights often reported as either being worn by the attacker or spotted nearby in the sea. This single attacker, who is the Big Sister, one of the Little Sisters from the first entry in the series, has mechanized herself in a similar fashion to the Big Daddies — and has become the ruler of the underwater city of Rapture, which under her control has reached an equilibrium in which she "actively maintains the city's infrastructure, and viciously hunts anyone down who disrupts the balance of power"; Tenenbaum, discoverer of ADAM and original creator of the Little Sisters, states that the Big Sister has begun kidnapping girls and turning them "into creatures...like her".
A subtitle, "Sea of Dreams," was supposedly dropped according to 2K Games Community Manager Elizabeth Tobey, before 2K withdrew the statement, stating that the "Sea of Dreams" subtitle would still be part of the full title.[ However, a later statement from 2K spokesman Charlie Sinhaseni stated that the "Sea of Dreams" title was for the trailer and not the game itself. The first major details on the gameplay and plot of the game were revealed in the April 2009 issue of Game Informer magazine, around the same time that the "viral" site "There's Something in the Sea" was revealed. This site documents a man's investigation into the disappearances of girls from coastline areas around the Atlantic, along with a mysterious red light that accompanies each kidnapping and pictures depicting what look like a big daddy. On April 9, 2009 on the Spike TV show GameTrailers TV with Geoff Keighley the first Bioshock 2 gameplay video was shown featuring the Big Sister. This demo showed many features including the ability to swim under water.
On May 8, 2009, it was confirmed that Digital Extremes would be producing the multiplayer component of the game.