Part of the joy of being an expert in your field is ripping Hollywood to shreds when it portrays something inaccurately. It’s what causes judges, detectives and police officers to shout “That would never happen!” when watching Law & Order.

Aquatics professionals rarely have that opportunity. So it’s a real treat when something like 12 Feet Deep comes along.

The premise is preposterous: Eager to close for the evening, the pool manager, played by the awesomely creepy character actor Tobin Bell -- better known as Jigsaw from the Saw franchise -- overlooks two girls struggling below the surface trying to retrieve an engagement ring from a drain grate. With a push of a button, a fiberglass cover rolls over the pool, sealing the sisters in a watery tomb. Making matters worse is a nefarious night janitor who torments the poor girls as they struggle for survival.

Let’s first address the star of the film – no, not Bell, who is criminally underutilized here. I’m talking about the pool cover. It’s a fiberglass contraption that rolls out from behind a wall. For what purpose would a cover like this serve? Does it turn this Olympic-sized pool into a giant dance floor? The movie doesn’t say.

“I’ve not seen anything like that,” said Rudy Stankowitz, president and CEO of Aquatic Facility Training & Consultants. And he’s been to every major university natatorium in the Southeast. “These pools don’t use covers,” he said.

John Fletemeyer, who has visited pools all over the world, has never encountered anything like it, either. In the unlikely event that an indoor pool of that size had a cover, it would be of a flexible material that floats on the surface to retain heat – not rigid fiberglass panels.

“It defies logic,” said Fletemeyer, executive director of the Aquatic Law & Safety Institute.

It’s also not very sturdy. The sisters escape (spoiler alert) by smashing their way out using the drain grate. (How they ripped the grate off the pool floor is another eye-roller.)

“Look at how easily it shattered toward the end," Stankowitz said.

So much for my dance-floor theory.

Another worthwhile observation from Stankowitz about the cover: "There is no way that would possibly ever meet any ASTM standards for pool safety covers used in the U.S.”

In one scene, the janitor threatens the girls with death by chlorine. She pushes a button on a control panel. Suddenly, the sisters are blasted by jets of chemicals. “That was awesome!” Stankowitz said with a laugh. “I’ve not seen anything like that in any of my travels that dispenses chlorine in that method or capacity.”

Another of the movie’s egregious errors: The janitor switches off the pool’s heater, threatening to turn the girls into Popsicles. The sisters instantly begin shivering. Keep in mind, the pool is indoors. With a cover.

In fairness, it’s probably a bit much to ask the filmmaker to understand the nuances of pool-cover mechanics and chemical feeders. But he should get basic math right … right? The trailer for 12 Feet Deep gives the pool’s ominous dimensions: “100,000 gallons of water. 50 meters wide.”

By Stankowitz’s estimation, the pool would actually be closer to a million gallons, unless it was a very narrow shape, which would be odd. That leads him to draw one conclusion: “They left a zero off,” he said.

By the way, 12 Feet Deep is "based on true events" .... whatever that means. Its official Facebook page points to one example in 2013 in which a woman drowned in a pool under a "protective sheet" after a night of drinking.

"The likelihood of something like that happening would be extremely remote," Fletemeyer said.

12 Feet Deep is currently available on iTunes, Amazon and other streaming platforms.