30 years ago ...Natalie Wood died by drowning and Originally, law enforcement believed Wood slipped off the cruise boat her and husbandRobert Wagner were on. But now could it be...Robert Wagner may have had something to do with it.

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.

The captain also claimed Wagner instructed the crew to NOT call in the Coast Guard to aid in search and rescue efforts once they realized Natalie had gone missing.

Natalie Wood's death has been reopened as a homicide investigation, primarily because of a book written by the captain of the boat where Natalie Wood spent her last minutes ... a book suggesting foul play ... this according to sources close to the investigation.

We've learned Marti Rulli -- who co-authored the book -- has been in regular touch with the L.A. County Sheriff's Dept. over the last few months.  She has given Sheriff's investigators documents supporting material in her book, and we're told the Sheriff is so impressed he has assigned 2 full-time homicide detectives to the case.

In the book -- "Goodbye Natalie, Goodbye Splendour" -- Rulli and Splendour Captain Dennis Davern write about the night Natalie drowned.  They say before Natalie disappeared from the boat, she was drinking and taking Quaaludes with her husband, Robert Wagner and actor Christopher Walken.

According to the book, Wagner became enraged when he saw Wood and Walken speaking, and smashed a wine bottle, yelling at Walken, "What do you want to do, f**k my wife?  Is that what you want?"

At that point, Walken returned to his cabin and Natalie and Robert went to their state room. According to the Captain, he heard a loud argument between the couple and thumping sounds, and eventually silence.

A short time later, the Captain went to the deck and was told by Wagner, "Natalie is missing."

The book claims Wagner refused to let the Captain call the Coast Guard.

Wagner told the Capt. the dinghy was gone, along with Natalie, but some doubted that because she was deathly afraid of dark water.

The death was ruled an accident, but Rulli was like a pit bull since the book was written in 2008.