Turkish-born Necati Izmir admitted that he left the bridge to go to the toilet two hours before the crash and had no recollection of what happened.
A crew of 10 were onboard the Antigua and Barbuda registered cargo ship Coastal Isle, which was heading from Belfast to Greenock when it grounded at Garroch Point on the Island of Bute in the early hours of 2 July, the Maritime and Coastguard Agency said.
Their report added that no-one was hurt and there was no pollution. The ship suffered serious damage but was refloated after around four-and-a-half hours and continued its journey.
Izmir was found in his cabin by a second officer who was sent to find him - ship captain Bernd Radel claimed he was found in his bed.
He pleaded guilty at the Sheriff Court in Greenock last November for failing to safely navigate the ship, and was fined £5,000 in a court in Scotland on Tuesday.
Captain Singh Grewal of the MCA said: "This was a very serious incident. It could so easily have been a dreadful tragedy but fortunately there were no injuries or pollution."