The crew regularly slept, slouched and listened to music on duty, investigators said
US Navy investigators have placed the blame for a March 2009 collision in the Gulf on "ineffective and negligent" leadership of the submarine involved.
Fifteen sailors were injured when the nuclear-powered USS Hartford collided with the USS New Orleans, a navy ship.
Sleeping, slouching and a radio room with music speakers were tolerated on board the submarine, the report says.
Five submariners would routinely sleep on watch, and the navigator took an exam while listening to his iPod.
The report was obtained by the Navy Times newspaper under a Freedom of Information Act request.
According to the report, the crew of the USS New Orleans - which ruptured an oil tank in the crash in the Strait of Hormuz near Iran - bore no fault for the accident.
Five known 'sleepers'
"Correction of any one of nearly 30 tactical and watchstander errors, or adherence to standard procedures, could have prevented this collision," said Adm John C Harvey, as he endorsed the investigation's findings.
The USS Hartford's commanding officer, Cmdr Ryan Brookhart, was relieved of his duties following the collision. Several other crew members were punished.
The report states that the submarine had five known "sleepers" - sailors who would routinely take a nap on watch. Two of the five were working when the collision happened, but there was no evidence they were asleep.
Helmsmen told investigators they would often "slouch in their seats with one hand on the controls".
However, the report said most of the crew were nevertheless of a superb calibre, and were eager to restore their vessel's standing.
The cost of repairs to the Hartford so far stands at more than $100m (£60m), while $2.3m (£1.3m) worth of damage was caused to the New Orleans.