You might call BioShock 2 'BioShock with the top down'. You'll discover this early in the game, when its lid, sides, and bottom suddenly disengage and you're thrust beyond the safety of purpose-made geometry. In BioShock, Rapture was a nightmare metropolis beneath the sea, an abyssal macro-terrarium circumscribed by an ocean unwittingly intruded upon, but for all the rifting city's dribbling cracks and spitting leaks, the ocean never came to you.
In BioShock 2, Rapture's reinforced glass and metal edges are only a measure of protection, a measure the design team seems fond of occasionally snapping out of true. Thank goodness you play a creature with proper accoutrements. The original's iconic renders of hulking creatures in monstrous diving suits with lighthouse-yellow blazing from their helmets? You're one of them this time, a Big Daddy, and instead of glimpsing them through bleary windows trundling along Rapture's perimeters, you'll follow in their footsteps through forests of coral, crabs, snails, and algae.