The Watchmen comic was (or, depending on your opinion, still is) famously considered to be unadaptable. The comic's creator, Alan Moore, certainly thinks so, and he's so against any of his comics being adapted that he refuses to be associated or even credited in any of them. You won't see his name on HBO's Watchmen, but the new series is less of an adaptation of the original 1986 graphic novel and more of a continuation.
As a result, there are a lot of references to the comic book that are left utterly unexplained (at least for now). The biggest of these might come partway through Watchmen's first hour. On the way home from school, protagonist Angela Abar and her son find themselves caught in what can only be called "a squid rainshower."
Watchmen is hard to adapt because, despite the cast of superheroes running around, this is not a "superhero" story. It's an alternate-reality fiction, with far more in common with Philip K. Dick's The Man in the High Castle than the Marvel Cinematic Universe. Superheroes are real, but that's just the start of the many uncanny differences between Watchmen's world and our own.
In Watchmen, America won the Vietnam War and made the country the new 51st state. Richard Nixon had Woodward and Bernstein murdered, and he remained president well into the 1980s. (Win some, lose some, I guess.) Also, the weather's different. Climate change is even more dramatic: it's raining squid, and no one seems to think this is weird in the slightest.
But these squid showers are actually a major callback to the Watchmen comic, specifically to the story's climactic ending.
**Spoiler Warning: There are spoilers for Watchmen below**
The Watchmen comic begins as a murder mystery. Someone killed Eddie Blake, who was once the Comedian, part of that Justice League-like group from the 1940s, the Minutemen. Times have been tough on superheroes, who were banned from operating in the late 1970s. Several members of the next generation of superheroes come together to solve Blake's death.
And then a giant, one-eyed squid falls from the sky onto New York City. It's impaled by the buildings and dies instantly, but not before sending out a psychic pulse, which kills some three million people.
This "alien monster" is said to have come through an interdimensional portal, an accident caused by government experiments conducted into portal travel through space. The discovery that humans are not alone in the universe causes everyone to lay down their nuclear arms. The U.S. and the U.S.S.R. make peace, global harmony ensues.
But it's not an alien. The giant squid was a hoax created by the genius industrialist/former costumed hero Adrian Veidt, aka Ozymandias. He staged the whole thing, a hoax, to fool humanity into world peace. The deaths of millions saved the lives of billions. The other costumed heroes who uncovered Veidt's plan after it was 35 minutes too late to stop it are horrified, of course, but they can't argue with success. Rather than risk a return to the Cold War (and a world of governments pissed at the antics of superheroes), they agreed to cover it up. All except one, Rorschach, whose self-righteous views cannot abide by such lies. But before he can tell anyone, the actually super-powered Doctor Manhattan kills him.
In the new Watchmen series, Rorschach has become a martyr, a symbol seized upon by right-wing extremists. But they don't seem to have any idea why he died. Like Angela and everyone else in Tulsa, they treat these squid showers as a byproduct of modern life, a reminder that humans aren't alone in the universe. At least these are small and relatively easy to deal with. No one dies, car washes make money; everyone wins.
But as comic readers know, it's a hoax. There never were aliens. So what, exactly, is falling from the sky? More importantly, how? Fans are pretty sure Jeremy Irons is playing Veidt, although HBO is being coy about explicitly confirming that. Assuming he is, though, that he's still alive, living in a castle. Did he create a machine that causes random squid to fall from the air? Are these a pointed reminder, in case humans start turning on one another again? (If Irons is somehow not Veidt, who among those who know the truth could be running this?)
It cannot be a coincidence this was introduced as part of the first episode. For 35 years, humans have believed in aliens and portals, and that the tragedy of November 2, 1985, was real. What happens if and when the truth of these squids comes out?