Rumours emerged last month (Mar16) alleging the Draft Day star had rescued spouse Christine Baumgartner from certain death after a great white shark lashed out at her during a snorkeling expedition near their home in Santa Barbara, California.

But unfortunately, only "half the story is true", according to Costner.

"My wife was on a paddleboard with a friend about seven months ago and a big 12-footer came at her about 30 yards off our beach...," he explained to Extra. "It came by her about a foot off the side. She said it displaced the water and and lifted her... and Christine went in to shore and was just really shaken."

Costner was nowhere near the scene of the scary incident - he was thousands of miles away in England filming his forthcoming action flick Criminal.

"She (my wife Christine) told me while I was in London making Criminal... which was really a helpless feeling," he noted of not being able to rush to his wife's side.

Although Christine is no longer swimming with sharks, Costner is concerned American presidential candidates are drowning in deep water.

The 61-year-old star is mortified by how some of the politicians in the running for the White House, which include Donald Trump and Hillary Clinton, have been using their platforms to discuss social issues.

"I find it embarrassing," Costner stressed. "I find it highly immature. I think America is really teetering at a low point with the way we talk to each other."

Costner's new crime drama Criminal, which also stars Tommy Lee Jones, Gary Oldman and Ryan Reynolds, will reach U.S. theatres on 15 April (16).