How about America gets the ability to launch bombers from aircraft carriers. (They cannot land on the carriers, but they can launch from them.)
This ability is only available in the Pacific and if at any time the American fleet is stronger in the atlantic than in the pacific, it is lost for the duration of the game. (Fighters are air assets, not naval assets, even on aircraft carriers.)
Regarding Ships. Ships are organized by size/cost. The more the ship costs the bigger the size of the mold/unit is.
The two that are hardest to distinguish is the cruiser and destroyer. The destroyer is the cheaper of the two, and therefore also the smaller of the two.
If you are confused - just line up one of each type of all the ships somewhere off to the side, so you can see the differences.