The tug Asterix had been moving the chemical tanker Donizetti off a berth at the Fawley marine terminal in March 2015 when it was pulled over sideways.
It capsized, trapping the coxswain in the wheelhouse, who was rescued by a colleague shortly before it sank.
The Marine Accident Investigation Branch (MAIB) found there were safety issues with how the crew were trained.
Investigators found the coxswain lost control of the vessel in strong winds shortly after 20:00 BST on 30 March 2015.
According to the report, the coxswain lacked training in the correct use of a rope -known as a 'gog', used to reduce the risk of the tug being pulled over sideways.
It also found "weaknesses in the communications" between the Donizetti and Asterix.
The report also identified launch crews were not sufficiently experienced in the use of the tug's towing hook emergency release mechanism.
Safety and training recommendations have been made to the tug's operator Ã˜stensjÃ¸ Rederi AS, to ABP Southampton and to the National Workboat Association.
Following the incident a crew member of the tug boat was given a bravery award for his actions in saving the coxswain.
Both men needed hospital treatment  for shock and hypothermia.
A report into how a tug boat came to capsize in Southampton Water has pointed to inadequacies in training.