This is quite an interesting situation. The normal game is as boring as the first one (didn't play 2 and 3, seems like I haven't missed much).
But the Sea stuff is amazing. The difference between the sea gameplay (interesting fights, tactics, multiple weapons that are very well balanced, actual participation in doing stuff) and the normal fighting (press E to win, button smashing while your char autoruns to victory) are so apart from each other that it's gonna be interesting for the future.
The thing is: ubisoft fucked up. If they would have created a new brand, they'd be the winner now. Now they are in this hilarious situation that they fucked up their main franchise. What are they gonna do next? Another Time period? I can't imagine ever playing another AC that has only the boring lame-ass "run towards target and play minigames on the way" gameplay. So they either find something to top the pirate gameplay, which will be damn hard - or go and make 10 pirate sequels. I can already tell it's #2. Either way they're fucked. Funny
If they would have created a new game, with more replayability, and even more cloned from Pirates!, then they would have had the chance to create a goty. Like this, it's a game that doesn't know what it wants to be - and still manages to be better than all ACs before combined.
But they really did their homework on the seafaring stuff. The feeling when a gust hits you suddenly from the side, the ship starts heeling, and you feel like you really have to push against it.. amazing. Well done. Just wished the wind would actualy be more accurate, but well, its still a console game. The thing that really made me cringe was the sails fluttering all the time