The "significant incident" happened last summer on a crossing from Arran with 82 passengers on board.
The second of four ordered ships will be handed over in October, about 20 months later than originally planned.
Police Scotland said a 23-year-old man was arrested and it was established that the threats "were not credible".
CalMac's west coast ferry network was badly disrupted by breakdowns earlier this year.
Final fitting out and commissioning work will take place in Greenock with the ship due to be handed over by the end of the year.
The ferry firm took the decision after a passenger filmed in a restricted area without permission.
The vessel will provide an additional 10,000 passenger spaces for the festival later this month.
The Turkish-built MV Isle of Islay has been unable to use its bow doors since mid-May.
CMAL has taken legal ownership of three ships as the shipyard faces economic challenges.
