Focus on the Story
That's what Billy Wilder's known for: story. The man was a master of his craft. And so it makes sense that the movie's production focused on the story, too. Everything we see on screen is in service of the strange, unsettling goings-on in the Desmond mansion.
Let's start with the look of the thing. Cinematographer John F. Seitz brought a dark film noir-style to Sunset Boulevard, like on Billy Wilder's earlier Double Indemnity (1944). True, the movie isn't about gangsters or a private eye, but it is about a murder, which complements that film noir feel and makes it seem appropriate.
As for Billy Wilder, well, he wasn't known for doing the kind of innovative camerawork that directors like Hitchcock could pull off. His focus was on using the camera specifically to tell the story—he wanted to highlight the writing and dialogue. But that doesn't mean he didn't know how to use the camera. He was no rube.
Getting the Shot
To the contrary, Wilder went to extraordinary lengths to get the underwater shot of Joe's floating corpse. It was really tough, apparently, but after several unsuccessful attempts, they managed to place a mirror on the bottom of the pool and film the reflection, obtaining the shot that way and achieving one of the more haunting and iconic images in the film.
P.S. Sunset won Best Art Direction at the 1951 Oscars—a tribute to its distinctive look.