Pictures and images emerged on social media of the orcas between Dunoon and Gourock on Saturday.
Lindsay Moss was on a Western Ferries service around 12.30pm when they made an appearance.
Her footage shows three killer whales, including what appears to be a youngster, rising to the surface before disappearing again.
Orcas are sociable animals and travel in groups of up to 50.
Despite their “killer” nickname, they have never been known to show aggression to humans in the wild and rarely among themselves, according to cetacean charity ORCA.
In the UK, they are seen most regularly around the Shetland and Orkney islands.