Essentially, Tales of the Black Freighter (along with Under the Hood and the behind the scenes footage) is either an epilogue for those who have just seen the movie or a preview for those who cannot await the Watchmen special edition DVD which will contain all three works (movie, Hood and Tales) all woven together the way they were in the book. As such, the price is a bit steep, which is the only reason I've rated this at four stars instead of five. Tales is beautifully rendered and voiced. Under the Hood is a bit slow, but then it was all text in the book, and in essence provided a break in the pacing of the story, so I'm not sure that this is a mark against it.
A note on Blu-Ray: Tales is supposed to feel like a comic book. The Animation harkens back to 1970s/80s fare such as Heavy Metal, and really doesn't benefit from the hyper-sharpened, line-for-line picture of my Blu-Ray copy. I'd actually suggest DVD format for this one if you have a less than 40" TV or a good upscaling DVD player.