The Flying Phantom capsizedin thick fog almost seven years ago while towing a cargo vessel on the River Clyde near Glasgow.
The incident happened six days before Christmas 2007 when the tug ran aground off West Dunbartonshire.
Captain Stephen Humphreys, 33, and Eric Blackley, 57, both from Gourock, died along with Bob Cameron, 65, from Houston, Renfrewshire. Brian Aitchison, 37, from Coldingham in the Borders, was rescued after he managed to cling to a buoy.
Port operator Clydeport Operations was fined during sentencing at the High Court in Edinburgh yesterday after pleading guilty to health and safety breaches at a hearing last week. The court held that the Clydeport failings were not responsible for the deaths of the sailors.
The tug operator, Svitzer Marine, last year admitted a series of health and safety breaches and was fined £1.7 million.