Well while we are on the subject of navies...
What about river going navies? Particularly in regards to the Mississipi and the Cival War.
Both sides had quite large fleets. There were iron clads, ram ships, transports, and huge battles. The river was used to supply troops and to transport troops.
You couldnt seige a fortress on the river unless you blockaded it with a navy.
The whole fortress vs ship aspect is cool too. Basically if an invasion fleet wanted to pass by a fortress they had to shoot it out with the fortress while they passed.
Somehow I don't think they are going to do it. They should have had naval movement along the Mississippi in EU2 even, as the Portugese and Spanish explorers sailed up it, and the British sailed down it in 1814 to attack New Orleans.
In the Cival War though control of the Mississippi was absolutely critical. Once the Union had control of it, not only could they run supplies and troops up and down it as they please, the Confederates could no long transport supplies or men across it. They basically cut a big chunk of the South right off.