The main thing I think you run into is that in 10 rounds, I don't know how you could find ways to muster 12 ships, and if you did manage it, a ship can't take a castle, so you will have spent 6-7 rounds making a navy, and have a footman or two protecting your lands.
You also run into some pretty heavy supply issues. There are only so many sea areas, so even if you managed to get six barrels(Highly unlikely if you are focusing on the sea, because apparently Westeros does't like fish and there are no barrels out there), that allows for a 4,4,3,2,2(I think). You would then have to take control of 4 sea areas and basically use all of your Army limit making boats that can't take castles required to win the game.
With that being said, the only balance issue i think you run into is that Greyjoy gains a huge advantage because his house deck is navy strong, and to a lesser extent Baratheon who has a pretty good boat card.
If you like sea battles, I would maybe say try it out with 8 ships and see what happens. Seems like a decent number, but I think 12 is probably too many unless you start adjusting a lot of game mechanics to make it work.
King in the North!!!!!!!!