The man fell into the water from the pontoons at Oban Marina on the island of Kerrera late last night.
He managed to clamber out onto a rigid inflatable boat where he was spotted by someone who raised the alarm about 11:30pm.
A local doctor, who happened to be close by, treated the man who was said to be in a “very cold and confused state”.
The Oban RNLI lifeboat was sent to the scene by coastguards and took the 74-year-old man to Oban harbour.
He was then transferred to an ambulance and taken to hospital, coastguards said.
A post on the RNLI Oban lifeboat Facebook page said: “UK Coastguard requested the launch of Oban lifeboat to an elderly gentleman who had inadvertently fallen in to the water from the pontoons at Oban Marina.
“After some time in the water the casualty had managed to get himself onboard a RIB tied up alongside, where he had been spotted, in a very cold and confused state.
“A local doctor who happened to be close by, initially tended the casualty.
“The lifeboat arrived on scene a few minutes after launching and the casualty was carefully transferred onboard, along with the doctor. Back at the lifeboats berth both the casualty and doctor were transferred on to a waiting ambulance.”