-No, its not really unique to CnC. A lot of RTS games set in the past have them (Empire:Total War, Age of Empires 2, Empire Earth...)
-It worked well because there was no real navy - there were just tanks that could move on land, tanks that could move on water and tanks that could move on both. Some of them just used ship models. It never came anywhere close to a realistic navy.
So yes, a navy works great if you make it totally unrealistic and simplified. Which is why it does not belong into a Generals game.
-Yes, back in WW2 Navy was quite important.
But in Generals2? As far as I know the game takes place if Europe, Asia or the Middle East. In a szenario like this, airpower would be far more important than ships.
And do you really think the GLA could build a navy that could compete with the USA or the EU? Or China?
Yes, thats called "marketing". I dont think they ever denied the Zombie General suggestion because they dont want to lose any potentional buyers. They propably would answer the same when asked for space battles....