You learn not to expect that much from a horror movie being released the first week of the year, when theater auditoriums continue to draw folks with higher-profile movies that hit on or around Christmas Day.

However, “M3GAN” — a tale of an artificial intelligence designed for children going epically out of control — arrived in that window last year with a fun mix of scares and laughs.

That provided hope that “Night Swim” — an offering from the “M3GAN” triad of Universal Pictures, Atomic Monster and BlumHouse washing into theaters this week — would be similarly entertaining.

But while “Night Swim” stays afloat for a while, it drowns in its third act.

The problem isn’t that “Night Swim” — written and directed by Bryce McGuire, who based it on his 2014 short film of the same name and shares the story credit with Rod Blackhurst — eschews the mildly campy tone that worked well for “M3GAN.” After all, a haunted backyard swimming pool is, as you come to observe, deadly serious business. But then it IS a problem that “Night Swim” scores so many, seemingly unintended, chuckles during its climactic stretch when things go off the deep end.

Until then, we get acceptable if by-the-numbers stuff, McGuire introducing us to the Waller family, led by former Milwaukee Brewers third baseman Ray (Wyatt Russell, “The Good Lord Bird”) and teacher Eve (Kerry Condon, “The Banshees of Inisherin”), the parents of teen Izzy (Amélie Hoeferle, “The Hunger Games: The Ballad of Songbirds and Snakes”) and her younger brother, Elliot (Gavin Warren, “Fear the Walking Dead”). They’re on the hunt for a new home and settle on a fixer-upper with a seemingly long-unused pool that appeals to Ray, whose career ended due to his battle with multiple sclerosis,

In fact, when we meet Ray, he’s battling headaches and blurred vision, his new doctor suggesting the MS has progressed significantly. In Eve’s mind, if there is any good that could come from this is Ray accepting that his baseball career truly is over. After years of trades and the resulting relocations, she’s ready to put down roots.

“I want that, too,” he says. “I promise.”

And yet, after the pool’s been rehabilitated, he seems to be benefiting miraculously from its spring water: A wound on his hand heals quickly, and he feels better than he has in a long time, with subsequent medical tests looking very promising.

On the other hand, the remaining Wallers have less triumphant experiences with the pool, especially when each goes for a … NIGHT SWIM! (Sorry, we couldn’t resist.) We’re talking spooky visions and worse.

Each Waller gets his or her own subplot. While the thread of the undersized Elliot struggling with the game that made his father famous doesn’t really lead anywhere, McGuire gets a bit more from the crush Izzy develops on a boy in the junior varsity swim club she’s joined. The game of “Marco Polo” they play in the pool — at night, of course, while her parents are away — is what passes as “Night Swim” at its frightening best.

Eve, meanwhile, sets out to investigate the pool’s problematic past by chatting with a former owner of the home (Jodi Long, “Shang-Chi and the Legend of the Ten Rings”), a conversation that doesn’t exactly go, well, swimmingly.

“Night Swim” has its effective moments. But by the time the water has had its way with Ray, putting the rest of the family in extreme danger, we’re already looking for a beach towel.

At points in “Night Swim,” when a Waller is night-swimming for his or her life while submerged in the magical water, the pool seems incredibly large and quite deep, occasionally almost infinitely so. We were never quite sure if this was a supernatural quality of the pool, as a character never references it, or merely a visual choice made to heighten the audience’s experience.

It matters not, as the increasingly wet affair is, ultimately, rather constrained and shallow.

“Night Swim”

Rated PG-13. At the AMC Boston Common, South Bay Center, Causeway, Alamo Drafthouse Seaport and suburban theaters. Grade: C