Scott MacAlister, 40, died when the boat he was skippering, Speedwell, sank off the island of Easdale on April 25. His body has never been found.
A Marine Accident Investigation Branch report said "it is almost certain that Speedwell floundered due to water ingress through the unsecured and unsealed flush-deck hatch to the aft store".
It added: "Speedwell's owner and skipper placed insufficient importance on safety and were prepared to accept and live with defects and sub-standard equipment."
The MAIB recommended that the boat's owner John Connell take steps to ensure that any vessel he may own in the future is operated safely.