Someone who survived a fall from a cruise ship at night would quickly become exhausted and cold, battered by waves they cannot see, an expert says.

Survival medicine specialist Dr Paul Luckin says fatigue would be the biggest challenge for a young couple who went overboard almost two days ago if they weren't injured by the impact of hitting the water.

Paul Rossington and Kristen Schroder were caught on CCTV falling from the cruise liner Carnival Spirit, around 8.50pm (AEST) on Wednesday, off the NSW mid-north coast.

"They would be extremely fatigued and fatiguing very fast," Dr Lukin, an adviser to the Australian Maritime Safety Authority (AMSA) and police rescue teams around the nation, told AAP.

"Being in the water at night is particularly difficult because you can't see waves coming.

"If you're stuck in the water and constantly being hit in the face by water, that is tremendously fatiguing."

Swallowing sea water restricts the airway and causes vomiting and diarrhoea, adding to dehydration.

"It will certainly be under three days that you would expect somebody to survive without water in those conditions," Dr Luckin said.

He estimated the water temperature where the couple went overboard would have been 21 degrees, while the human body is 34 degrees.

"Even in relatively warm water, they will become hypothermic," he said.

All of these factors old contribute to a person's ability to stay awake and alert in the water.

"Once you do become fatigued, then it becomes increasingly difficult to keep your head above water," Dr Lukin said.

An ocean search is a highly technical exercise requiring huge amounts of behind-the-scenes work, he said.

Experts use the estimated point of entry and data on tide, winds and water currents to determine a drift path.

"It's not just a matter of sending somebody out to have a look," Dr Luckin said.