Operators P&O and Stena had to be alerted by coastguards to ask them to proceed with caution as the man was directly in their path on Wednesday.
The 70-year-old man suffered a head injury after he tried to launch an inflatable from his motorboat and lifeboat crews were scrambled to rescue him, along with a Royal Navy helicopter on exercise nearby, and the P&O fast-rescue craft.
It emerged the OAP was not wearing a lifejacket and was using paddles made from old oil drums.
A passing motorist had spotted the incident and contacted 999.
People are now being warned to be aware of their surroundings when out on the water, and to take the necessary precautions.
Coastguard watch manager Peter Stewart said: "We do not want to discourage people from enjoying the good weather we have seen recently, but if you are enjoying a day on the water you must always remember to use the appropriate equipment and always wear a lifejacket.
"This man went out in an old boat with paddles made from five-gallon oil drums to attempt to test out a children's inflatable. This is a dangerous loch, which also happens to be a busy commercial port and ferry path.
"It is entirely possible that, if it were not for the informant who saw the man in difficulty on the water, that today could have ended in tragedy - be it through the man's head injury, the onset of hypothermia or a collision with any one of the commercial or passenger vessels that operate in the area."
A coastguard spokesman said: "At 2.45pm on Wednesday afternoon, Clyde Coastguard received a report via Liverpool Coastguard that a motorist had sighted a man in difficulties in Loch Ryan.
"The man had reportedly gone out into the loch in a 12-foot motorboat along with a toy inflatable that he then tried to climb into, which then capsized. When he then tried to pull himself back into the motorboat he suffered a head injury.
"At 3pm the inshore lifeboat arrived alongside the casualty who had since climbed into his motorboat, but was unable to row back to shore due to his head injury and shock from being submerged in the loch."
The incident comes five years after a father and his two sons were killed in the loch when the boat they were in capsized, drowning Shaun Ridley (37) and sons Steven (15) and Michael (12), on holiday from Manchester.
A probe revealed a freak wave had caused the boat to turn over but the source of the wave was unclear.
The same year, a canoeist died in the River Bladnoch, near Wigtown, and an angler's body was found off the rocks at Kirkcudbright.