Another, even more dramatic image, appears to show a wall of water suspended in midair about to engulf one of the outdoor decks.
The Manly Ferry, which travels from Circular Quay, adjacent to the Sydney Opera House, to the seaside suburb, is a key link in the city's transport network and a must-experience tourist attraction.
Usually a placid ride, the trip can get choppy as the ferry passes between the North and South heads leading out to the Pacific Ocean.
One person commenting on Mr Gilchrist's photo said: "On big days we used to get rain gear on and go out on the bow as she crashed through the Heads. Waves crashed all the way up to the upper windows and bridge.
"Don't think they allow that anymore."
Despite the waves, the Manly Ferry service completed its journey without incident.