The Stratagems Coastal Cities Guide

To gain a Coastal City...you need to seize a Coastal Plain....Coastal Plains are much more difficult to seize than regular plains...even lvl 0s and 1s can have a mixture of a couple thousand troops...including fighters, bombers,destroyers, subs, and battleships.

Tip: Scout for a lvl 0/1 Coastal Plain with only submarines on it...thesse are the easiest to seize. Subs are weak against ground troops, and can easily be taken by a tank/rocket attack.

Having a Coastal City you will have a additional building not available to inland cities....the Naval Academy. This will allow you to build Destroyers, Submarines, Battleships, and Aircraft Carriers. Also this will be the easiest way to build Special Forces troop. There are many Missions that can only be completed by having a Coastal City.

I currently have 2 Coastal Cities. I do not have a Warehouse nor a Trade Center is either, so I can make room for more Arms Plants and Houses. I also recommend in new city that you get 1 of each resource field to lvl 9 asap for reseaarch purposes. 

Here is what is needed to make all ships and Special Forces.

Naval Academy (NA); Heavy Manufactoring Plant (HMP), Light Manufacoring Plant (LMP), Academy (AC), Arms Plant (AP), Airport (AR), Steel Plant (SP).

The Naval Academy requires a HMP6 before you can build the NA. This will also allow you to research Heavy Industry tech to lvl 6.

Destroyers - NA 5, AP 8, SP 4, Steelmaking tech 4, Heavy Industry tech 5
Submarines - NA 7, AP 8, Steelmaking tech 4, Heavy Industry tech 7
Battleships - NA 9, AP 9, Steelmaking tech 6 (need SP 6), Heavy Industry tech 10 (need HMP 10)
Carriers - NA 10, AP 10, SP 10, HMP 10, RC 9, Steelmaking tech 10, Heavy Industry tech 10
Special Forces - AP 9, AR 9, LMP 10, RC 9, Training tech 10, Deployment tech 10