In twos and threes, the young men strolled down the narrow streets of Bahrain's old commercial center on a languid October evening.

Though they tried not to be conspicuous, the sailors of the U.S. Middle East force were given away by their accents, short haircuts and off-duty uniforms - T-shirts, blue jeans and, here and there, a baseball cap with "USS Vandegrift" spelled out in gold letters.Until recently, shore leave was unusual for U.S. Navy men in the Persian Gulf, who kept a wearying vigil along the entire waterway during the climactic months of the Iran-Iraq War. Only since a cease-fire took hold Aug. 20 has the Navy relaxed enough to allow its men ashore in the ports that dot the Arab side of the gulf.

On a recent weekend in Manama, the steets were thronged with young sailors, not only American but also French, enjoying the relatively cosmopolitan atmosphere of a place where liquor is available at hotel bars and women go about without being swathed in black.

According to Navy officials in Bahrain, the relatively relaxed atmosphere reflects the altered strategic situation in the gulf, where no ship has been attacked since the cease-fire began. The speedboats that Iran used to harass shipping have either stayed in port or steered clear of neutral shipping.

After two months without a serious incident, U.S. officials said recently that they were now most concerned about the possibility of floating mines breaking loose in the northern gulf and drifting into the sea lanes where huge oil tankers move into and out of Kuwait and Saudi Arabia.

A Navy spokesman said that "a lot of these floaters" were being found in the northern gulf and quoted a ship commander as remarking that "the mines don't know there has been a cease-fire."

"It's a heightened interest for us," the spokesman said. "Otherwise, our friends are going to find these things washing up on beaches or wedged against a pipeline."

Of immediate concern is the likelihood that the high winds and heavy seas that buffet the area in December will cause the mines, which are generally on the surface or just below it, to tear loose from their mooring chains.

Many of the mines are situated in Iraqi or Iranian territorial waters near the mouth of the waterway called Shatt al Arab. They were sown there at the start of the war in order to disrupt Iraq's oil exports. Each country will have to remove the mines or arrange to have them removed, Navy officials said.

Because of the mine threat and the fragility of the cease-fire, the Reagan administration has announced that there will be no sudden U.S. withdrawal from the area.

"We will maintain a responsible level of vigilance as long as there is a threat that hostilities can erupt," U.S. Ambassador Sam H. Zakhem said in an interview. "We are more at ease now than before. But we have not and will not put our guard down."

There have been minor changes in the deployment of forces, however. In July 1987, the United States announced a policy of escorting American-flag vessels through the gulf to protect them against Iranian attack. The policy was principally of benefit to Kuwait, which re-registered half of its 24 tankers in the United States in order to make them eligible for Navy protection.

Last month, the Reagan administration quietly implemented a change in the escort policy, giving up the close escort - a warship in advance and to the rear of a line of merchant ships - and substituting a form of "zone protection."

A Navy spokesman said a U.S. warship now needs only to be "in the vicinity" of an American-flag ship.

"It gives us a little more flexibility," the spokesman said, adding that American-flag ships are still being escorted through the Strait of Hormuz, the narrow waterway that links the Persian Gulf with the Arabian Sea.

U.S. force levels are still about what they were at the height of the Iran-Iraq conflict, with 16 warships in the gulf and 10 others in the Arabian Sea.

A spokesman said the crews are still at the state of readiness known as "Condition 1 Alpha." This means that guns are manned and missile launchers are at the ready, so that the ships can react quickly to any threat. Crews are on a schedule of six hours on, six hours off, "which gets very tiring," an officer said.

The only significant change in ship assignments came when the guided missile cruiser Vincennes was withdrawn from the gulf and the Pentagon canceled plans to replace it.

It was the Vincennes that on July 3 shot down an Iranian jetliner over the gulf, killing all 290 people on board. The incident occurred immediately after an engagement involving the Vincennes and elements of the Iranian navy, and it raised questions in Washington about the performance of the cruiser's missile defense systems.

U.S. officials said that although the size of the Middle East Force may be reduced as the threat diminishes, the United States still has a commitment to countries in the region to maintain a permanent presence in the gulf, as it did before the Iran-Iraq War.