Alex Brizard deliberately did not buy a beverage when he went to see a 9:30 a.m. showing of “Avatar: The Way of Water” this month. It did not matter: He still had to run to the bathroom three times over the course of the movie’s three hours and 12 minutes.

“It was honestly all the water,” said Mr. Brizard, 29, who works in finance and lives in Scottsdale, Ariz.

The sea gushes, gurgles and sprays across the screen in James Cameron’s “Avatar” sequel, which immerses viewers in the richly textured aquatic world of a clan of reef-dwelling Na’vi. These photorealistic ocean sequences make up the bulk of a lengthy epic that many are watching in theaters with vats of soda in hand, creating a perfect storm for moviegoers to need to take one or more bathroom breaks.

The water scenes are especially vivid in IMAX 3-D, according to Mr. Brizard, who moved to an aisle seat after his first trip to the restroom so he wouldn’t have to step over other audience members on subsequent visits. “All the splashing noises, it’s all a constant reminder,” he said.

Mr. Cameron, who has encouraged fans to see “Avatar” in theaters, saw this coming.

“Here’s the big social paradigm shift that has to happen: It’s OK to get up and go pee,” he said in an interview with Empire magazine. He added that he did not want anyone “whining about length” when people are perfectly willing to watch eight straight hours of television. (Television, it must be said, is easier to pause than a movie in a theater.)

In November, Mr. Cameron told The Hollywood Reporter that moviegoers should go to the bathroom “any time they want” during the movie. “They can see the scene they missed when they come see it again,” the director added. Representatives for Mr. Cameron did not respond to a request for comment on Thursday.

Getting the water exactly right was one of Mr. Cameron’s preoccupations. To ensure that it appeared appropriately aqueous, he submerged the film’s actors in a 32-foot-deep tank that held about 90,000 gallons of water. Cast members got scuba certifications and trained with a free-diving instructor, and the movie’s sound mixer kept four microphones constantly running in the tank to capture underwater background noises.

Mr. Cameron’s attention to detail may be part of the reason many theatergoers have reported having to take bathroom breaks. In a 2015 study, the sound of running water increased the flow rate of urination among men with lower urinary tract issues. The sound of running water is also a commonly recommended component of toilet training for infants.

“The theory is, mainly it’s just a conditioning phenomenon,” said Dr. Kent DeLay, a urologist at Lexington Medical Center in West Columbia, S.C. Since the sound of water is so often present in the bathroom, many already associate it with urination. Among Dr. DeLay’s patients who already experience incontinence, urinary urgency can be worsened by just the sound of running water. “I would think that the visual would have some impact as well,” he added.

Like Mr. Cameron, Dr. DeLay encouraged moviegoers to step away as necessary in order to avoid damage to the neural pathways around the bladder. “For the most part,” he said, “when you have the urge to void, you should.”

Many are taking his advice. Gene Naumovsky headed into a movie theater on West 34th Street in Manhattan with a combination meal of a flatbread pizza and a large Sprite.

“At first I thought I’d kind of see what happens and, you know, ride the wave,” said Mr. Naumovsky, 23, an executive assistant. “But an hour and a half in, it was an inevitable problem.”

He made a break for it when the movie’s protagonists, Jake Sully (Sam Worthington) and Neytiri (Zoe Saldaña), greeted the water clan, because he expected a lengthy training montage rather than a major action sequence or a plot twist.

Matthew Aryeh, 26, a lawyer who saw the movie at a theater in Nassau County, on Long Island, said he had often insisted on rewinding sections of movies at home that his family had talked over.

But during “The Way of Water,” which he described as “long and slow at times,” he left for a minute-and-a-half bathroom break. His brother took one about half an hour later.

In response to the film’s length, some have suggested bringing back the intermission, which was a common feature when movies were screened using multiple reels of film. Intermissions were included in later lengthy epics including “Gone With the Wind” (3 hours 42 minutes) and “Lawrence of Arabia” (3 hours 38 minutes).

Mr. Naumovsky said “The Way of Water” would have benefited from a brief intermission. Mr. Aryeh disagreed.

“I think a better fix for this movie would have been to just shave 45 minutes off,” he said.