Really not bad at all! I'm a father of three sons, a 10 year old, a 12 year old, and a 13 year old.
One day they all come to me asking if they could get this game, now as most parents would, I said no because it was an M rated game. They still asked me so I looked into the game, It's got some swearing and violence, but not bad at all. You can turn off Blood and Voice volume, so it could easily be a Teen or almost E10+ Game.
If you're considering to get this for yourself, or your kid, then don't hesitate! It's a great game! You're a pirate and my sons love the Pirate Of The Caribbean series so they love the fact that you can have naval wars, sneak around, and it's not all about the violence either, you can always run away from fights too. I've played a little bit of this game and the fighting is a bit challenging at times, but it might just be me. But overall my recommended age for this game is 10+.