A workboat which sank after hitting a Thames passenger vessel set off without proper checks, the Marine Accident Investigation Branch (MAIB) said.
The crew of the Alison, a cruise firm's supply boat, were rescued from the water after the crash with the Typhoon Clipper last December.
The catamaran was carrying 48 passengers, but nobody was injured.
The MAIB said the Alison's crew "headed out into the river without properly assessing the shipping situation".
Its report said they had also failed to make their intentions clear to other vessels and Typhoon Clipper's master did not see the Alison before the crash near the Tower of London.
This was because the workboat was initially obscured by a pier and then moved into his blind spot.
The collision occurred at 11:00 on 5 December last year, shortly after both vessels departed Tower Millennium Pier.
The crew were treated for symptoms of cold shock and admitted to hospital for checks after the collision.
MAIB recommended that the Port of London Authority review its guidelines for boats with limited visibility.