I rented the disc, and I agree that "Under the Hood" was great. It's about 32 mins long. Definitely the better of the two features. I expected it to be just thrown together, but in fact it's actually quite well produced. It's much better than you're typical DVD documentaries. If you've read the book, you'll love it. If not, some of the stuff might fly over your head.
"The Black Freighter" is okay. If you've read the book, it's pretty much what you'd expect. Nothing spectacular, but the animation is quite nice looking. It's only a brisk 20 mins. It kind of fleshes out some of the details that are left out of the book. When I read the book, I always sped through those parts, because I found the Freighter sub-story kind of boring.
I read that these two features were originally going to be edited into the main movie, however, after watching, I can't imagine how that would be possible. At first I thought Snyder left them out for time constraint reasons, however, it would be really difficult to fit them in the movie. I think they stand alone just fine. I can see them included them on a final Watchmen ultimate edition disc, but I don't he'll ever edit them into the movie.
One more thing... Even thought I liked both features on this disc, I would not recommend buying it right now, mostly because it's only about 50 mins long, yet they are charging full price for the disc. $20 for the dvd and $25 for the blu is ridiculous.