The moments of humor before this flip seem unintentional, such as when a character explains with grave intonation that Jonas (Jason Statham) actually can swim without a pressurized suit underwater at the bottom of a 25,000-foot trench, as long as he controls the air pressure in his sinuses — that’s just Statham logic. It’s not until we’re out of the trench and up on the surface that director Ben Wheatley loosens up and has a little fun.

But that first half is dire straits. Wheatley takes the helm from John Turteltaub, who directed the delightfully ridiculous original “The Meg” in 2018, and works from a script by the first film’s writers, Jon and Erich Hoeber and Dean Georgaris (both thrillers are based on books by Steve Alten). Wheatley and the screenwriters gloss over their setup and pay not even a passing interest in the new crew aboard the research vessel Mana One, where interchangeable supporting actors deliver sarcastic quips with a strangely flat affect. When a few of those new faces meet their watery graves at the bottom of the titular trench, the only possible reaction is, “Who?”

Jason Statham in the movie “Meg 2: The Trench.”

(Courtesy Warner Bros. Pictures/Warner Bros. Pictures)

The film’s focus is on hero Jonas and the teen in his care, Meiying (Shuya Sophia Cai), the daughter of scientist Suyin, who featured in the first film and is now mysteriously dead. Suyin has been replaced by her brother, Jiuming, played by Wu Jing, one of the biggest action stars in China. (The “Meg” films are U.S.–China co-productions and cater to both audiences.)

The ebullient Jiuming is a daredevil marine biologist who has taken up his sister’s mission to protect the oceans through underwater exploration, and he’s also taken a special interest in training and befriending Haiqi, the young megalodon living in the care of the Oceanic Institute. (As a quick reminder, megalodons, or “megs,” are massive prehistoric sharks that live in a trench at the bottom of the ocean, contained by a thermocline barrier).

As explorers are wont to do, they poke around in the trench, and what do they find? Capitalism: There isn’t an inch on this planet that some craven rich person won’t figure out how to strip mine. While this trench sequence apes the underrated 2020 Kristen Stewart vehicle “Underwater,” the secret mining operation serves as the plot wrinkle to throw our leads into danger, reveal a few characters as nefarious, and pierce the thermocline barrier, freeing the prehistoric apex predators.

Wu Jing in the movie “Meg 2: The Trench.”

(Warner Bros. Pictures)

The whole gang heads to “Fun Island,” which is an apt moniker for the second half of this movie. On the beach, all manner of ancient sea creatures terrorize clueless tourists, and Wheatley lets it rip, taking the opportunity to riff on other classic creature features like “Predator” and “Jurassic Park.” He also nods to Renny Harlin’s “Deep Blue Sea,” with Kennedy delivering LL Cool J-esque one-liners (as well as a howler of a shark-themed rap song for the credits).

In the crystal clear waters of Thailand, serving as Fun Island, Statham delivers his quota of outlandish action sequences, battling a Meg with only a Waverunner, a jury-rigged spear and his signature snarl (his straight-faced performance lends to the camp, both intentional and unintentional). Wheatley also unleashes his not-so-secret weapon, Jing, a charisma bomb and true wild man, who has an approach to stunts that rivals Jackie Chan and Tom Cruise. He fights kraken and amphibious dinosaurs gracefully; he flings himself onto and off of helicopters with gleeful gusto; he’s willing to take a pratfall and let his stunts be a punchline too. His unfettered verve and humor is much needed after the questionable first hour.

It’s an odd viewing experience, to have the second half of a movie not necessarily redeem the bland first half, but rather gain the nerve to be what it wants to be, leaning into the slippery silliness of a summer shark flick. With a blue drink in hand and movie-theater air conditioning blasting like salty sea gusts, there are worse ways to spend an August afternoon.

Katie Walsh is a Tribune News Service film critic.