'Deadliest Catch' Deckhand Mahlon Reyes Died of Cocaine Overdose

Mahlon Reyes, a deckhand on one of the "Deadliest Catch" crab fishing boats, died from a drug overdose ... TMZ has learned.

Mahlon's cause of death is listed as acute cocaine intoxication ... according to Flathead County Sheriff and Coroner, Brian Heino. He also tells us the death has been ruled accidental.

TMZ broke the story ... Mahlon died in Whitefish, Montana back in July, after suffering a massive heart attack and going on life support. His wife told us his death came as a complete shock to the family because he had no known existing health conditions.

Mahlon's death came while he was rehabbing a torn Achilles, suffered while crabbing, and we were told he was working his butt off to make it back to Alaska for another crab season.