Tales of the Black Freighter is a dark, morbid story of a man's descent to madness, full of corpses and death and symbolism. It parallels Ozymandias' journey to the same place.
The Under the Hood documentary is Hollis Mason's take on the events of the world of Watchmen. It is self-indulgent and a little pretentious, but so is Mason.
This is a companion to the Watchmen DVD. If you are a Watchmen fan and you have the DVD of the movie then you must get this set. If you aren't and don't then do not buy this one.