THIRSTY as always for still more subsidies and greater federal protection, the maritime industry is back with its oil bill. This insatiable coalition of companies and unions wants Congress to pass a bill requiring that 30 per cent of all imported oil arrive in American-flag ships. President Carter is reported to be having trouble making up his mind about it.

This bill would impose a heavy new tax, indirectly, on people who buy oil and gasoline. This hidden tax would go to the sole benefit of a small industry already lavishly subsidized. There's nothing wrong with taxing oil to raise its price and discourage consumption. But it ought to be done openly, with the money used for the public's benefit. This bill would impose the tax invisibly, by forcing oil importers to use the most grossly overpriced shipping in the world.

The most credible estimates suggest that the bill would cost consumers at least several billion dollars a year. Currently, nearly all imported oil arrives in ships that are registered abroad - even if they are owned by American companies - to avoid excessively expensive U.S. requirements. Despite all the government aid, U.S. shipping survives only where foreign competition is excluded by law.

The maritime lobby, never subtle, has hired Gerald Rafshoon Advertising, Inc., of Atlanta. That's the same Gerald Rafshoon who worked for the Carter campaign last year. The Rafshoon firm has been running ads in newspapers, including this one, warning that foreign-flag tankers "threaten our environment, weaken our national defense, and take away U.S. jobs."

In fact, the U.S. Coast Guard has broad powers to set safety standards for all ships, foreign as well as American, coming into U.S. waters. National security is not a real issue; the threat to national security is the present degree of American dependence on foreign oil wells, not the flag under which the tankers sail. As for jobs, it's hard to think of any industry in which several billion more dollars a year would. Create fewer of them. Wages are high on the big tankers, and automation is far advanced.

It's a strange commentary on American folkways. Congress seethes with indignation at Mr. Carter's modest plan to tax gasoline for a public purpose and rebate the revenues to the public. But the same Congress solemnly considers - and may even pass - a bill like this one, taxing the oil covertly for the sole benefit of one industry that already enjoys more different kinds of subsidies than any other in the United States. Meanwhile, a President dedicated to fighting inflation can't decide whether to support the bill, which everyone agrees would be inflationary. Strange, isn't it?