well.first of it needs power and since it it's an underwater city you might be able to harvest heat and gas from underwater volcanic vents. Wow great idea
the city will also need clean water so a facility will be needed to take salt out of the water and store it. wow good idea again!!
oxygen is also a must so it will need plants that can grow via artificial sun lights. Artificial lights, oh yeah
the city will also need a reason to be underwater,it might be harvesting a special deep sea ore. It is actually there to mine water and search for life (Europa is a m00n of JUUPPPIIITTTEEERRR, had to say that sorry)
a trade place like a train or boat will also be needed so the city can export and import what it produces and needs. I will add a landing pad to top of elevator