While some said it was "dazzling", others described it as a "soggy, twee, trillion-dollar screensaver".
The Way of Water is the follow-up to 2009's Avatar, which remains the highest-grossing movie of all time.
BBC reports Director James Cameron has suggested the sequel needs to become the fourth-biggest film ever just to break even.
In The Way of Water, viewers are transported back to the planet of Pandora, where the existence of the Na'vi people is once again under threat.
In a one-star review, The Telegraph's Robbie Collin suggested watching the film "feels like being waterboarded with turquoise cement".
"The story, cooked up by Cameron and a four-person writing team, is a classic piece of franchise-elongation, in which nothing meaningful happens or important changes, and all the pieces are returned to their original positions, ready for the next instalment," he said.
But Empire's Nick De Semlyen took the opposite view, awarding the movie five stars and describing it as "thunderingly entertaining".
"It's a leap beyond even what [Cameron] pulled off with the first film, a phantasmagorical, fully immersive waking dream of a movie in which something impossible is happening on-screen at almost every moment," he said.
Others took issue with the film's length. "At over three hours, you will have time to quietly mull geopolitics, as well as holiday plans, meeting schedules and recipe ideas," joked Danny Leigh of the Financial Times.
The first Avatar film saw disabled marine Jake Sully (played by Sam Worthington) infiltrate the planet's indigenous population by taking on an avatar body.
He ultimately defected to join the Na'vi species himself to defend the planet against an attempt by humans to obtain a precious mineral.
The Way of Water is set many years later. Jake now has a family with Neytiri (Zoe Saldaña), and the film sees them join forces with a reef tribe as new threats to Pandora emerge.
The Guardian's Peter Bradshaw was not keen on the film, likening it to a "trillion-dollar screensaver".
Awarding the film just two stars, Bradshaw said: "The story, which might fill a 30-minute cartoon, is stretched as if by some AI program into a three-hour movie of epic tweeness."
Also opting for two stars, The Times' Kevin Maher described it as a "visually spectacular but superfluous" sequel.
He suggested lead character Sully is "monotonal" and said the film felt repetitive, adding: "Everything here is retread, restate, regurgitate."
"Avatar: The Way of Water has scenes that will make your eyes pop, your head spin and your soul race," wrote Variety's Owen Gleiberman.
But, he added: "There is almost zero dimensionality to the characters. The dimensionality is all in the images."
The Hollywood Reporter's David Rooney said the film "delivers on action, emotion and thrilling 3D visuals".
"In terms of narrative sophistication and even more so dialogue, this US$350m sequel is almost as basic as its predecessor, even feeble at times, he wrote.
"But the expanded, bio-diverse world-building pulls you in, the visual spectacle keeps you mesmerised."
The Independent's Clarisse Loughrey landed in the middle with a three-star review, headlined: "You probably still won't care [about Avatar], but at least it's very pretty."
"These films, which are largely identical in tone, don't feel like acts of love, but of material achievement," she wrote. "They exist more to be respected than to be adored."
There was another three-star review from Metro's Larushka Ivan-Zadeh, who said: "Whilst there's always tonnes of action going on, it's remarkably hard to engage with it, or care much about these bland characters.
"As a spectacle it's astonishing. But is this even a film?" she asked. "It's more like a three-hour theme park ride."
Justin Lang of the LA Times was more enthusiastic, suggesting the film "will cure your moviegoing blues".
"Cameron pulls you down so deep, and sets you so gently adrift, that at times you don't feel like you're watching a movie so much as floating in one," he said.
Returning to Pandora "just became the best deal on Earth for the price of a movie ticket," agreed IndieWire's David Ehrlich.
He described the film as "rapturous and awe-inducing", adding that the viewing experience "simply isn't comparable to whatever else is playing" in cinemas.
Elsewhere, Digital Spy's Ian Sandwell said it was a "visual masterpiece that's often beautiful to behold, but one that doesn't always have the substance to go with its considerable style".
Awarding the film four stars, the Evening Standard's Charlotte O'Sullivan wrote: "Plot-wise, this movie is treading water. But that's fine, because the water's lovely."
The Daily Mail's Brian Viner was also positive, concluding: "This sequel is tremendous fun, even bigger and better than the original, but by golly it will test your bladder."