1. The Heroes of WATCHMEN are based on "real" superheroes
While people unfamiliar with Watchmen will probably think of characters like Nite Owl and Dr. Manhattan as simply veiled versions of Batman and Superman, this is really only indirectly true, in that Superman and Batman were the archetypes for hundreds of superheroes to follow. In fact, the WATCHMEN heroes are based on the Charlton Comics super hero pantheon. DC bought the Charlton brand and was looking for a way to use the characters, but the proposal Alan Moore brought to them would have rendered these bought and paid for characters unusable for future projects, so Moore was forced to come up with imitations. In fact, the heroes are alternate versions of: Captain Atom (Dr. Manhattan), The Blue Beetle I and II (Nite Owl I and II), The Question (Rorschach), The Peacemaker (The Comedian), Nightshade (The Silk Spectre) and Peter Cannon, Thunderbolt (Ozymandias).
2. The Heroes of WATCHMEN are based on OTHER "real" superheroes
While the Charlton heroes created the template, some of the other aspects of the characters are based on heroes outside the Charlton Universe, including Superman (Dr. Manhattan), Batman (Nite Owl), Mr. A (Rorschach), The Shield (The Comedian) and The Phantom Lady (The Silk Spectre).
3. Alan Moore does not want to be associated with the WATCHMEN movie
OK, you probably did know that, but what you don't know is WHY. It's only partly about an objection to crass commercialism. In the beginning it goes back to the deal that Moore had with DC Comics. If the Watchmen characters were not used by DC within a certain amount of time, the rights to them were to revert to Moore. Since there would be no reason to use the characters once the series was completed, it seemed a fait accompli that Moore would get the characters back. However, to sidestep this loophole, DC had the main character in their book "The Question" (The Question being the character Rorschach was based on, see above) READING the graphic novel and DREAMING about the characters, as a way to retain the copyright. Needless to say, Moore was less than pleased.
4. There ARE no "Watchmen"
In the movie, "The Watchmen" are referred to as if they were an established superhero team, featuring Rorschach, Dr. Manhattan, Nite Owl, The Comedian, the Silk Spectre and Ozymandias as a sort of alternate reality Justice League. In fact, in the graphic novel, only one super team exists, the Minutemen, who were disbanded in 1949. There is a gathering of the "Watchmen" for one ill-fated meeting in the late 60s in a failed attempt to create a team, but they are referred to only (and only by the would-be founder, Captain Metropolis) as "The Crimebusters."
5. There ARE no superhero comic books
In the movie, after Ozymandias reveals his master plan too late for anyone to do anything about it, he scoffs at Nite Owl that he is no "comic book villain." In the graphic novel, Ozymandias says he is no "Republic serial villain," referring to an old movie company that would run film series of western heroes, super heroes, and other B movie fare. In addition to the fact that this sort of sounds cooler, Ozymandias does this because in the book, there are no more super hero comics, since with the existence of super heroes, this type of entertainment would no longer be escapist fantasy. Instead, comic book stories consist of adventure subjects like pirates and cowboys, and in fact, a pirate comic book features significantly into the plot of the graphic novel.
Hope this enhances your enjoyment of WATCHMEN