Keira Knightley as Lo in "The Woman in Cabin 10".

In the new Netflix movie, “The Woman in Cabin 10,” Laura “Lo” Blacklock wakes in the night on a luxury superyacht to the sound of a woman screaming. The journalist, played by Keira Knightley, opens her balcony to see what looks like someone going overboard and blood smeared on the partition between her veranda and the room next door.

She alerts the crew, which sets off a frenzied search for the apparent missing guest – but everyone seems to be accounted for. When a crew member directs another to cancel their mayday call to the Coast Guard, Blacklock responds, “You called the Coast Guard?”

“Of course we did,” he responds. The ensuing murder mystery – based on Ruth Ware’s book of the same name – is full of high-drama Hollywood moments, but that is part of real-world maritime emergency protocol.

Here’s what happens if a murder really occurs at sea.

What is the standard protocol for a missing passenger?

If a guest or crew member is seen going overboard, the vessel would immediately initiate overboard protocols, “which is basically – the easiest way to say it is, turn around the vessel to go back exactly where the person was and look for them,” Michael Winkleman, a maritime attorney with Lipcon, Margulies & Winkleman, P.A., told USA TODAY.

However, if such an incident was not observed or caught on camera, they would look for the person onboard first. “So, if they're not seen, then they will initiate a search of the vessel so that they don't expend the huge amount of effort to turn around and look for someone who is actually sitting in the bar having a cocktail,” he said.

That can take hours. If they don’t turn up, the crew will likely turn the vessel around to search for them while also notifying the relevant nearby authorities.

What happens if someone is murdered at sea?

Like the yacht in “The Woman in Cabin 10,” many vessels have onboard security. “And when they've got suspected foul play or any kind of a crime, they are the police force,” said Winkleman. The standard protocol would include quarantining the scene and working to preserve any evidence in the area.

“If they have a suspect, (the security personnel) can take that person into custody, keep them in custody – basically put them in jail on the ship,” he said. They would alert relevant authorities in those cases, as well, including at the next port of call.

“So if they were on the way to the Bahamas, they would notify the (Bahamas) Maritime Authority,” Winkleman added.

Who has jurisdiction over crimes at sea?

That depends. For example, the FBI’s investigative authority and ability to enforce U.S. laws aboard a seagoing vessel are determined by where it is at the time, who owns it, its flag state (where the ship or boat is registered) and more.

“Generally, the U.S. has jurisdiction over crimes committed on a ship if the crime occurs within the maritime jurisdiction of the United States and the vessel is owned, in whole or in part, by a U.S. person or company,” the agency’s website reads. “Additionally, there is jurisdiction over crimes committed by or against a U.S. national on board a foreign vessel if the (crime) occurs during a voyage having a scheduled departure from or arrival in the U.S.”

Winkleman noted that jurisdiction in the case of homicides or other crimes on the high seas can be “murky.”

“But it's not that different than if you're in Florida and you have a car accident, and you've got state patrol and you've also got sheriff's authority, and then you could also have some type of federal authority involved.”

Nathan Diller is a consumer travel reporter for USA TODAY based in Nashville. You can reach him at ndiller@usatoday.com.

This article originally appeared on USA TODAY: 'Woman in Cabin 10' features an at-sea murder mystery. What really happens in those cases?

Reporting by Nathan Diller, USA TODAY / USA TODAY

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