This is a very interesting scenario, there are so many things to consider:
There is hardly any need for streets, bridges etc. Everything and everyone is able to "fly" (swimming but in 3D) around as they wish. @TheDemonLord mentioned ocean currents but this does not seem very efficient since the would have to be created artificially to be really useful and then it is just easier to propel humans and submarines directly instead.
The need for structures is also much lower. There is no weather to protect from and the temperature is always the same (4°C) which is the preferential temperature for your humans anyway since they evolved in it. So the only reason for structurs is maybe the need for privacy and protection from noise. Basically all public events could just take place outside.
With the right amount of air containers, structures are effectively weightless so they are a lot less massive than above-water structures since they do not have to support their own weight. They would probably look more like zeppelins anchored to the ground (or not anchored, it is not that difficult to move them around). The dimensions of the bouyancy creating upper part and the lower living part would be inverted (big living part, small bouyancy part). Pressure is not a problem, since the inside pressure is the same as outside.
Everything will take place in 3D including traffic. The city will be 3-dimensional, going vertically is basically the same as going horizontally, so traffic (swimming humans, later submarines) would probably just move all over the place in the beginning and will have to be restriced to lanes later when speed and density increases. This might look like this:
(Except for the buildings)
Moving around heavy objects will be much easier since everything can be weightless with the right amount of bouyancy created by air tanks. The term submarine for your vehicles might be misleading, they do not need to be massive at all. A lot of the personal transport would probably take place on scooter-like vehicles since there is no weather and hence nothing to protect from except the driving "wind".
Sewage systemy might be challenging. I assume a pressure based system like it is used in space could work. If your looking for more inspiration for details, solutions for living in space might be helpful because a lot of challenges there are similar to your cities.
I cannot stress enough how challenging the 3D part is. All cities we know are essentially 2D regarding most of the planning. Your cities can be entirely 3-dimensional which creates a lot of interesting situations for your planning and stories which may be hard to convey to your readers but very great if it works.