I don't know exactly what the calculation is (or where to find it), but I can share my strategy for getting aquatic cities moving fast.
1. At first only build/purchase buildings that will help with moving. My personal choices are: Recycler, Drydock, Water Refinery and Therm Rudder (also the Drone Command if/when you get that). The one exception is the Trade Depot which I always build first in almost every city. Once I have spending money I will purchase the first 3 buildings which allows the Trade Depot to be ready in 2-3 turns, then the Therm Rudder is usually around 4-6 turns and I'm ready to move.
2. Move until you've covered the entire area you plan to work for this city. If the city is taking more than 2 turns to move, send more trade routes.
3. Once it is in its final resting place, go crazy. Depending on the atmosphere, I will prioritize defense buildings since water cities are super vulnerable without them.