Flames ripped through the site after the fire started in a car workshop at Medina Village, Cowes on Monday.
Forty business units were evacuated along with residents in nearby streets.
Isle of Wight Fire and Rescue Service said early inquiries revealed a garage worker had attempted to put the fire out using a handheld extinguisher.
No-one was injured.
Classic ships, racing yachts and carnival floats built to mark the 120th anniversary of Cowes Carnival charity in the summer were destroyed in the blaze.
Ferry services to the island were also stopped.
The fire service said a full investigation into the cause of the fire was still being carried out.
A fire on an Isle of Wight industrial estate started accidentally when welding sparks came into contact with fuel, a fire service has said.