Natalie Wood May Have Struggled for Hours Before She Died

Natalie Wood was wearing a red down jacket when authorities discovered her body 30 years ago ... and her wardrobe could prove she floated alive in dark water for hours while Robert Wagner allegedly stalled rescue efforts.

Marti Rulli -- the author of the book that triggered cops to re-open the investigation into Wood's death -- performed a forensic test on down coats similar to Natalie's ... which proved a down jacket acts like a life preserver in water ... "it will not sink, but instead remains floating."

Rulli's finding completely contradicts the opinion of the coroner in the case -- who insisted the jacket would have sapped Natalie's strength and weighed her down in the ocean.

Wood's body was discovered in the ocean off Catalina Island -- hours after she went missing from a cruise with her husband Robert Wagner and Christopher Walken.

Originally, law enforcement believed Wood slipped off the boat while trying to tie down a dinghy. But now, cops have doubts ... after the captain revealed Wood and Wagner had been arguing intensely before she went missing.