Implementing multiplayer might be relatively easy, but that isn't all you have to do for multiplayer. You also have to make sure it actually works, make sure that it is balanced and fun, make sure there's actually something to do in it.
UA:D will probably have lots to do with friends, if you each play as different countries in a campaign, but Balance is going to be worrying, as might making multiplayer actually work. For two examples, look to War Thunder and From the depths. War Thunder has a pretty horrible time trying to balance historical elements against gameplay elements because, in theory, each country has to be balanced against the others to be fair to the players of each nation. There isn't supposed to be any ludicrously overpowered things at any particular tier. I am personally pretty sure that they haven't achieved this. It's an extreme example and isn't anywhere near a 100% perfect analogy, but it makes me somewhat nervous for anything historical or pseudohistorical trying to balance multiplayer.
From the Depths Has had multiplayer since relatively early alpha. However, since there are no servers and it isn't particularly polished (which seems to be quite a likely scenario for UA:D), there are very many glitches such as things not being in the same place for both players and frequent crashes.
I think that it's probably a lot easier to simulate WWII era naval technology. A lot of it might have to be abstracted away and folded into tangentially related technologies, but let's use an example.
If we have carriers on the field of battle,
we need another hull.
Aircraft would have to have a finite amount.
The aircraft weapons would have to be ammunition.
Aircraft would have to somehow be recovered after attacking.
I've certainly missed a few things out, but these few points only seem to have one major problem with them, which might be solved by treating aircraft as vehicles from a mothership and Carriers as being aircraft ports; Perhaps abstract this as embarking new aircraft at sea when close enough to friendly ports?
This seems a lot easier than trying to make multiplayer work and be balanced to me.