Soviet submarines often surface in the vast natural harbor here to float lazily under the relentless sun for a few days before shoving off to resume underwater patrols in the Indian Ocean.

A half-dozen miles down the coast, at Aden's Khormakar Airport, Soviet reconnaissance planes take off on daily flights over the Persian Gulf and transports routinely airlift cargo to Socotra, an island 200 miles offshore where Soviet submarine tenders are reported to moor.

The military facilities afforded the Soviets here, although described by diplomats as limited, illustrate the strategic value to Moscow of this dirt-poor little nation that has brought the Arab world its only authentically Marxist government. But, according to Arab and foreign observers, the Soviet stake here is ideological as well as strategic, a chance to influence overwhelmingly conservative Arab societies with down-home communist doctrine.

There is no comparable Western presence. South Yemen has had no diplomatic relations with the United States since 1969.

Most of the region's Marxist-oriented opposition groups--including Omanis, Somalis, North Yemenis, Saudis, Iraqis and Egyptians--have operations here to channel South Yemeni or Soviet aid.

But since the pro-Soviet faction in the South Yemeni leadership won ascendancy two years after independence from Britain in 1967, most of Moscow's effort has been concentrated right here. The 24,000-man armed forces are entirely Soviet-equipped and, according to informed diplomats, Soviet advisers are assigned to most units of the Army, Navy and Air Force.

An estimated 1,500 Soviets work here on military or technical advisory jobs, these sources say, along with about the same number of Cubans, who are working in hospitals, and the Popular Militia and East Germans working in the State Security apparatus.

In "each corner, in each place, there is a Russian," said one Yemeni disenchanted with the government. "I am not sure, but sometimes I think even the leaders' speeches are written by Russians."

During a recent closely controlled visit here by a pair of American correspondents, Soviet and East European residents were seen only occasionally in the street or passing by in buses or military vehicles. Yemenis reported that the advisers sometimes encounter hostility in Aden shops when they bargain too hard, but Soviet policy appears designed to keep Soviet residents separated as much as possible in special housing compounds.

President Ali Nasser Mohammed's two-year-old government cloaks the exact number of Soviet advisers and their role here in secrecy. One foreigner who visited a Soviet-built fish canning factory reported, however, that the total staff of about 65 included 15 Soviets.

Despite the Soviet presence--symbolized by red stars lining traffic dividers or looking down from the volcanic peaks dominating Aden--Mohammed has taken steps to allow resumption of limited private commerce. Reversing the dogmatic state-run system of his predecessor, Abdul Fattah Ismail, Mohammed loosened import restrictions and permitted merchants to buy consumer goods from a state agency and sell them at retail in Aden's traditional hole-in-the-wall shops.

Trade and Supply Minister Ahmed Obeid Fadhli said in a written response to questions that such private commerce must play "an important role" in improving living standards here. But he estimated it has accounted for only 4 or 5 percent of the country's trade during the last several years.

Nevertheless, residents here report food and consumer goods that were impossible to find three years ago are now readily available. Souq Tawil in Aden's crater district appeared well stocked with food, toiletries and household goods during a correspondent's recent visit. But at least a third of the shops on Martyr Madram Street in the nearby Maala district remained shuttered even during business hours.

"You see this can of juice?" asked an Arab official, holding up a can of apple cider imported from Austria. "Five years ago you would not have been able to find it. They tried to impose change on the people by force, but traditions are just too strong here. Things are better now."

Mohammed, 43, has long been regarded as the most pragmatic of South Yemen's top leadership. Until he took over from Ismail, he had ranked as number two in the government, making his mark as an administrator.

His relatively moderate attitude is thought by diplomats here to control the government now, although Aden's recent history has been marked by erratic twists and turns. But in part, they add, the pragmatism is encouraged by lack of money, exacerbated by floods this spring, that has turned South Yemeni attentions toward the wealth of Saudi Arabia and its conservative Persian Gulf allies.

"They need cash, and they're only going to get it from the Arabs," a foreign observer said. "A greater feeling of realism has entered. It had to. And it has strengthened the hand of people like Ali Nasser Mohammed, who believes you can't eat dogma."

Despite more than a decade of Soviet aid, South Yemen remains visibly poor. The government recently announced plans to raise the annual per capita income--about $210--by half in the next three years.

It is unclear how this can be done without huge infusions of aid. Aden's main moneymaker, its port, has been in the doldrums since closure of the Suez Canal in the 1967 Middle East war caused a shift in sea traffic patterns. Its oil refinery, with 30-year-old technology, had been running far below its capacity of 165,000 barrels a day until recently, when Iran and Iraq began using it to replace their own war-damaged facilities. But informed sources here said it still is running at less than 120,000 barrels a day.

Against this background, Mohammed has been making tentative overtures to the conservative Arab neighbors whose cash he needs. Most efforts have concentrated on convincing Saudi Arabia that Aden can get along with North Yemen, its neighbor to the northwest, and Oman, its neighbor to the northeast.

The Marxist government for years has backed opposition guerrillas in these two countries. But there have been contacts with President Ali Abdullah Saleh of North Yemen and Sultan Qaboos of Oman in an effort to end the conflicts. Kuwait and Saudi Arabia have been involved as mediators.

Mohammed and Salah met in May to discuss technical aspects of a unity agreement and since then there have been numerous contacts among lower-level officials. But Western observers said there is no indication that any major breakthroughs are imminent.

Years of hostility and mutual subversion have made these efforts difficult. After agreeing several times in the last decade to unite and after exchanging presidential visits during the past six months, North and South Yemen drafted a joint consititution under Saudi and Kuwaiti patronage.

Yet sources here said the two nations almost went to war three months ago as North Yemen mounted a major military campaign against the South Yemen-backed National Democratic Front guerrillas seeking a change in the Saudi-influenced government in Sanaa. In addition, South Yemen executed 10 persons and jailed three in May on charges that they infiltrated from North Yemen to commit sabotage on instructions from Saudi Arabia after receiving training from U.S. and British experts in Saudi Arabia.

Similarly, agreements on reconciliation with Oman have fallen through. A meeting set for June 5 in Kuwait between South Yemeni and Omani envoys was canceled. In an apparently related visit, Prince Nayef, the Saudi interior minister, traveled here June 7 to see his South Yemeni counterpart.

According to diplomats stationed here--who say they have limited access to government thinking--the talk of unity and reconciliation inevitably runs into a barrier posed by South Yemen's fundamental Marxist orientation and its alliance with the Soviet Union and Moscow's other regional allies.

South Yemen signed a 20-year friendship and cooperation treaty with the Soviet Union in 1979. Later that same year, it joined in a treaty with the Marxist government in Ethiopia, and last August Libya joined in a three-way pact aimed at countering U.S. influence in the region.

With Oman offering military facilities to the United States and Saudi Arabia, a close U.S. ally, exercising predominant influence in North Yemen, observers here said, South Yemen's alignment with the Soviet Union casts a dark cloud over any chances for genuine good neighborliness in the region.