Yeah, it's good, isn't it? A lot of people didn't like it because it wasn't "Assassin's Creed" enough... too much pirate stuff. But considering they never advertised it as another romp through Italy, and it's not like they hid how much sea-based content there was, I don't think that's a fair critique. It has a few issues here and there, but it was a very good game, in my opinion. Maybe not my favorite in the series, but it does rank above Revelations and AC2, in my opinion.
I didn't really like the naval aspect of AC3 either. In fact, I didn't care for that much of AC3. The frontier was too large with too little to do, and the story was kind of boring. I also didn't like Connor much at all. Dude was a bit of a dick. It's kind of sad that I liked a templar more than the "good guy". Haytham was a badass.
But in AC4, the entire game is basically one big open world map. There are a few "cities" like Havana and Nassau, which allow for the traditional series gameplay, and there are still churches and stuff to climb. The high points on the lesser islands are usually done by climbing trees or the broken masts of ships.
And yes, there is whaling. You get in a little whaling boat and throw harpoons at whales and sharks. Actually, it's kind of awesome. Moby Dick (because seriously, let's call it what it is...) is the toughest whale in the game.
Yeah man, the game is pretty damn gorgeous. It's my favorite AC since Brotherhood (even though the climbing is still sloppy like it was in AC3). I love all of the other improvements they made though. It's also friggin' MASSIVE. I've played it for over 30 hours and last I saw, I was 75% complete. :/