But they are also a nationalised concern - formed in 1973 when the Caledonian Steam Packet Company and MacBrayne were merged. Since then, CalMac has enjoyed a virtual monopoly of Scotland's west coast sea routes.
The Government is now considering privatisation and a review is likely to be announced within a fortnight. A Scottish Office source said it would be a first- principles exercise, examining whether the company should remain in the public sector.
Any privatisation moves would be greeted by an immediate protest campaign. The island of Barra is served by four sailings a week in winter, six in summer. Its people fear this could be drastically reduced after privatisation.
Jessie MacNeil, secretary of the Barra and Vatersay voluntary council of social services, said: 'Flights to Barra may shortly stop due to new aircraft not being able to land on beach runways. So we would be even more dependent on the ferry. If the sailings came down to one a week, doing business on the mainland would become impossible.'
CalMac runs 32 ships from 51 terminals in 23 islands. Last year it carried 6.3 million passengers, only marginally down on recent record years. This year it will receive pounds 7.1m in government subsidy. Ten years ago the subsidy was pounds 6.5m.
'If it had kept pace with inflation, we should now be getting pounds 12m,' said Colin Patterson, CalMac's managing director.
But the grant level is not the whole point. CalMac depends heavily on a culture of cross-subsidy - routes which contribute to the company's profit back up those that do not. Ullapool to Stornoway on Lewis 'contributes'. So does Oban to Mull, the short Skye crossing. So do most of the Clyde coast routes.
Most other sailings, especially those to remoter islands such as Colonsay and Barra, would not be attractive to private operators.
Kevin Bryne, who runs a hotel on Colonsay, fears the end of subsidised links with the mainland. His solution? 'If the Government thinks it can't afford us, then let's look at other countries who can. Ireland? Norway? It might come to that.'
Not everyone is so full of praise for CalMac. James Mundle, who runs a road haulage company in Tarbert, Argyllshire, spent pounds 270,000 last year on crossings to Islay, carrying everything from whisky barrels to food. He described the company's attitude as often 'take it or leave it'. Discount rates for business? 'They won't budge,' he said.
Other business users, less willing to be named, say that CalMac seems to want to run bigger and bigger boats with fewer crossings. One described a government review as 'bloody good news'.
CalMac's two main arguments for remaining a monopoly - cross-subsidisation and the ability to offer back-up services - was challenged by Andrew Wilson, a director of Western Ferries, a company likely to pitch for franchised routes.
Western operated sailings to Islay between 1968 and 1981 before being forced out by CalMac. Mr Wilson claims that in 1975 - 'the last time I looked' - Western could have taken the subsidy given to CalMac for that route 'and run the service with passengers travelling free'.
Brian Wilson, Labour MP for Cunninghame North, and a shadow transport spokesman, said of the Government's review: 'Like mangy curs, they keep returning to the same old bones for another gnaw. Dismantling CalMac has been around since the early Eighties. It has never found any basis of support and has been rejected three times.'
The renewed debate over CalMac comes as the Government wrestles with the unpopularity of trying to privatise British Rail. The land battle appears about to spread into troubled Scottish waters.
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