Water can be seen in nearly every single scene of the film whether it be rain water, dark water, humidity, or household appliances such as a toilet, bathtub, or a washing machine. Because water is in nearly every shot, producer Bill Mechanic has said that "Water is one of the lead characters in the movie".
The characters of the young Dahlia and Natasha Rimsky are played by the same actress Perla Haney-Jardine. This was supposed to show that Dahlia symbolically saw herself in Natasha as they were both young girls who were neglected by their mother (or in Natasha's case both her parents).
(at around 55 mins) When Dahlia moves her wet clothes from one washing machine to another, you can see the number, "666", pop up for a moment on the second washing machine. That number is known as the symbol of the Anti-Christ.
(at around 19 mins) One of the main plot points of the film is Dahlia and Kyle's divorce, however, the reasons for said divorce are only implied. Kyle finds Dahlia to be mentally unstable and Dahlia suspects Kyle of having an affair, evidenced when Dahlia claims that Kyle wants to live in Jersey City because 'she' is there, 'she' referring to his new girlfriend.
John C. Reilly and Tim Roth both have roles in the Marvel Cinematic Universe with Reilly appearing as Corpsman Dey in Guardians of the Galaxy (2014) while Roth appears as Emil Blonsky in The Incredible Hulk (2008). Jennifer Connelly also appeared in a Hulk adaptation playing Betty Ross in Hulk (2003) while Perla Haney-Jardine appeared as Penny Marko in Spider-Man 3 (2007).
Dark Water was released in two different versions for the home media release; the theatrical cut and an unrated cut. Since its release rumours about said versions began to spread. On the internet one can find numerous reviews in which the writers can only consternately remark that they just can't recognize any difference between the two versions. Some reviewers even claimed that the two versions were identical, and others again that the unrated version only included more swear words.
The trivia items below may give away important plot points.
The film became notorious for its coincidences with the mysterious death of Elisa Lam at the Cecil Hotel in 2013 (8 years after this film's theatrical release). In the film the water becomes black because of a decomposing corpse of a girl in the water tank. Elisa Lam was found dead because of drowning in a water tank and was only discovered because the residents complained of dark water that tasted funny. Cecilia (Ariel Gade) shares a name with the Cecil Hotel where Elisa was found dead and Dahlia (Jennifer Connelly) shares a name with The Black Dahlia who was last seen at the hotel shortly before she was found dead. The film also frequently depicts elevator malfunctions, the story of Elisa became famous on the internet because of a video that depicts the last moments of Elisa in an elevator that malfunctioned. The first time Cecilia and Dahlia visit the apartment Cecilia sneaks off to the rooftop where the water tank is situated which baffles the characters because the door was locked and she somehow managed to get up there. A peculiar factor surrounding Lam's death was the fact that the door leading to the rooftop with the water tank was locked with only employees having the key to access it raising the question as to how Lam got up there in the first place.
In many scenes with the character Veeck (Pete Postlethwaite), gallons of water can be seen with him, in his apartment and the office, indicating that he knows not to drink the water in the building due to being contaminated by Natasha's corpse.