Not a song, a background theme.It's playing in the pool hall scene early on.Very familiar as a famous cop show theme. Maybe NYPD Blue.
It's called Canção Do Mar, and the show you're thinking of is Southland.
I just posted about this, before I saw this posting! Sort of answers what I asked. Looks like someone else noticed!
His a Portuguese song, actually. When I saw the movie I was like: "They actually implemented a Portuguese song in a movie!"