Dead Narrator Floating in Swimming Pool
It's not every movie that begins with its main character and star facedown in a pool. But then again, Sunset Boulevard is not every movie.
When we first meet Joe Gillis, he's already dead—his corpse is floating in Norma Desmond's swimming pool, after she shot and murdered him. The shot from the bottom of the pool, looking up at the floating corpse, is an iconic image—one of the most famous features of the movie as a whole.
In the voiceover, Joe, narrating the story from beyond the grave, comments on the irony of his own fate: "…The poor dope. He always wanted a pool. Well, in the end, he got himself a pool—only the price turned out to be a little high." An in-ground pool—this symbol of wealth and status—ends up being the site for Joe's death, after he failed to attain that wealth and status himself.
A bummer end for a bummer dude.
The Pool Itself
Earlier in the film, when we first see the pool, it's empty and has rats crawling around in it. The huge pool, languishing in this state of decay, encapsulates the destroyed glamor of Norma Desmond's silent-era Hollywood. Joe says, "And, of course, she had a pool. Who didn't then? Mabel Normand and John Gilbert must have swum in it ten thousand midnights ago, and Vilma Banky and Rod La Roque. It was empty now… or was it?" (After the "or was it?" Joe notices the rats—not exactly desirable swimming partners.)
But, apparently, taking Joe as a lover brings Norma back to life, and in her excitement over the movie she thinks she's starring in, she has the pool cleaned and refilled. Joe sticks with Norma because it gives him a chance to enjoy fine things (like the pool) and be pampered with gifts—even if he's effectively exchanging sex for money. Of course, by the time his corpse is found floating in the pool, we're betting he's not enjoying those benefits much anymore.