Bathysphere

From BioShock Wiki

The Bathyspheres of Rapture are the equivalent of very small, comfortable submarines, allowing travel between Rapture's different zones. They were part of the city's transportation network, the Rapture Metro.

Andrew Ryan closed off all access to the bathyspheres and the people protested to get out, obviously to no avail. Throughout the story, the bathyspheres are used by Jack as a mode of transportation to travel around Rapture and its "abandoned" city sections. Jack can use the bathyspheres, however, because his genetic signature is close enough to Ryan's that the security system doesn't deny him access.