This bleak, waterless rock and sand outcropping in the Arabian Sea may one day be required as an advance base if the new U.S. Rapid Deployment Force is ever called out to defend the Persian Gulf area.

But if Washington planners intend to use it as more than a mail-sorting stop, there is work to be done -- a lot of it.

Masirah is one of four air bases in Oman where the facilities would be made available to U.S. forces under still-secret terms of an agreement worked out by representatives of the Sultan of Oman and the Carter administration in June. Along with a bigger, but similarly barren desert airfield near Oman's border with Marxist South Yemen, it would be the closest American steppingstone to the gulf, completing a chain of regional facilities that could be used under similar agreements with Kenya, Somalia and possibly Egypt.

At present, however, the island has barely enough support facilities to handle Oman's small, 600-man aviation training center and its single squadron of a dozen light and outmoded jets based alongside a modest concrete airstrip.

Recently, base authorities were hardput to find sleeping quarters for 11 members of the U.S. Navy whose helicopter from the Indian Ocean fleet was forced to take shelter on Masirah during a storm, according to Gordon Brown, a British wing commander, who is on loan from the Royal Air Force to Omani Sultan Qabus ibn Said to command the training center.

The island is 44 miles long and ranges in width from about 18 miles to only a few miles at its narrowest points.

During a tour of the island base recently, Brown estimated that the cost of bringing Masirah's facilities up to the standards required by heavy U.S. jets and cargo planes would be $250 million.

That amount, which he said Oman eventually intends to spend upgrading the base and improving its now almost nonexistent defenses, would escalate "to an astronomical figure" if construction were done on a crash basis within the next year.

The U.S. Indian Ocean fleet now makes minimum use of the Masirah base as a weekly ferry stop for mail and provisions.

Although Navy surveillance planes frequently fly over the island, none land because their crews of 14 to 18 men are too large for Masirah to handle, Brown said. They land instead at Seeb International Airport, which serves the capital city of Muscat, 180 miles north of here.

Aside from the storm-grounded helicopter crew, the only American who has landed on Masirah lately has been Gen. David C. Jones, chairman of the Joint Chiefs of Staff, who visited Oman Oct. 2 and 3. Brown said Jones stopped off for an hour to look at the island while en route by helicopter to the fleet, where he paid a morale call on the crews of two American aircraft carriers.

One of the greatest drawbacks to Masirah as a military base, noted centuries ago by the Roman historian and nationalist Pliny the Elder, is that the island has no fresh water. Pliny called it the Island of Turtles because it is the main breeding ground of the giant loggerhead sea turtle, which outnumbers people on Masirah by more than 10 to 1.