Five of the crew members were apparently found above the drink-drive limit
CalMac ordered the whole crew of MV Isle of Arran to be breathalysed at Port Ellen on the Isle of Islay.
Local people said they were told that sailings to the mainland on Tuesday morning had been cancelled for "technical reasons".
One, Paul Graham, was waiting to board the 7am sailing to Kennacraig in Argyll when it was cancelled.
He said: "At 6.35am, I noticed a chap in a white shirt wander into the office and come out with a bag and realised it was a breathalyser going on to the vessel.
"Next thing, the marshalling staff said there was a technical delay and engineers were working on it.
"Then, about 6.45am, the chap had gone back into the pier office and a few moments later the staff approached vehicle drivers and said he was sorry, the vessel was not sailing for technical reasons."
More than 20 vehicles were stranded and unable to cross the water
He added: "There have been instances before when crew members have been breathalysed and suspended but to my knowledge it hasn't stopped a vessel sailing.
There were about 20 cars and a couple of trucks waiting to go on
"You can lose one, but if you lose five crew then than kind of number does affect the operational ability of the vessel. There were about 20 cars and a couple of trucks waiting to go on that ferry. To say that we were not happy is an understatement."
Islay McEachern, chairman of the Islay Ferry Committee, said: "We heard it was technical difficulties but then this rumour came out."
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The group emailed CalMac saying they had been informed "several crew members had been found to be above the permitted alcohol limit".
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A CalMac insider said: "It is rare for crew members to take a drink on board because the consequences are so severe..
"Almost everyone I know has a breathalyser to make sure they are never over the limit.
"There are no warnings of dismissal, quite rightly. We wouldn't allow someone to be in charge of a car while under the influence of drink or drugs. Why would we allow anyone to be aboard ship and under the influence?"
Locals reported that they had been told the ferry was cancelled for technical reasons
A CalMac spokesman said: "We can confirm MV Isle of Arran was taken out of service yesterday due to an HR matter.
"Formal investigations are under way and it is inappropriate to comment further on that matter. We regret that some passengers experienced inconvenience.
"As a responsible ferry operator and employer, we take the safety of our passengers, crews and vessels extremely seriously."