I didnt really care for this part in the graphic novel, but I saw this short film recently. It's violent and bloody, but other than that there's nothing objectional. Unless you hate violence, anyone over 10 should be fine watching it.
If you liked the Watchmen book or movie, watch half of this
I did enjoy the "Watchmen" book and I'm planning to see the movie. If you've read the graphic novel, you'll recognize the "Black Freighter" story as having appeared a piece at a time - and it was much easier to take that way. The animated feature is thoroughly disgusting. I recognize that "Watchmen" itself is very dark and violent, but it takes a sick mind to enjoy a feature like this one.
That being said, if you enjoy the super-hero story, rent this one for the "Under The Hood" segment. It's a "60 Minutes" style documentary about the characters from the movie that will give you some insight into their history, and is worth watching.