Industry, food, and power
"Everyone has to make money but making your own is frowned upon. Go figure." - Corin Hayes
The lifeblood of the Corporations is money. Lots of it. The more the better. The service industry still makes up a good portion of each corporations economy, but manufacturing has seen a resurgence. It is hard to live in the oceans when they provide nothing but easy chances to die.
Each factory is usually set a little away from the city, connected by walkways when close, or by sub when further away. They draw their power from the city supplies usually, but larger factories are self-sufficient.
Making any and everything, some factories are almost entirely operated by SI and robots, but there are jobs for people to. Some are the simple cleaning and press--a-button type, but many are related to the maintenance of the machines, the computers, and a thousand other little things.
Power / Energy
Nuclear Fusion is old-reliable tech. Except when it goes wrong. Then it is just a big hole in the seafloor - Corin Hayes
Most cities rely upon Nuclear Fusion for their power requirements. However, tidal propellers and geothermal energy is also employed by many of the larger cities.