The Reagan administration, in a significant change of policy, has decided to emphasize the sea as its military staging area in the Persian Gulf region if there is a flare-up there, rather than count on some friendly bordering country to serve as the launching pad for troops and equipment.

Pentagon officials said they have turned away from the idea of stockpiling tons of gear on land and will go lighter in that regard than the Carter administration had planned, counting largely instead on cargo ships and the British island of Diego Garcia to keep troops supplied for the first month of any combat in the region.

Critics contend this change of emphasis in Persian Gulf war plans is part of a much larger policy shift by the Reagan administration away from land forces and toward sea-power projection by the Navy and Marine Corps. Some predict the eventual result will be less investment in North Atlantic Treaty Organization forces, to provide more sea-based forces for trouble spots as far apart as the Persian Gulf and the Caribbean.

Champions of this emphasis on sea projection counter that they are just being realistic. They contend a confrontration with the Soviets or their surrogates in one place would mean potential confrontation everyplace, putting a premium on mobile troops, like Marines, and high-speed cargo ships.

The argument has been raging behind closed doors at the Pentagon as the fiscal 1983 defense budget, the first one the Reagan administration will write from scratch, goes into its final stage. A recent meeting of the Defense Resources Board, for example, hotly debated whether the administration should publicly commit itself to "maritime supremacy," sources said.

Not only are Navy Secretary John Lehman and Adm. Thomas B. Hayward, chief of naval operations, allied on that question, as would be expected, but so is the head of what is called the Pentagon's "little State Department," Francis J. (Bing) West Jr., assistant secretary of defense for international security affairs. His strong views on the advantage of sea-projection forces have earned him the nickname of "Maritime Bing" among some Pentagon staffers fearful that land forces are about to get short shrift.

Although an admitted sea-power enthusiast, West has no intention of turning away from the agreements the Carter administration negotiated to use land bases in Oman, Egypt, Somalia and Kenya in an emergency, according to defense officials. However, they said only a minimum of military gear will be stored on land in those countries.

On Africa's east coast, the Pentagon's emphasis will be on improving two ports, Berbera in Somalia and Mombasa in Kenya, with $25 million and $50 million respectively to be spent on those projects this fiscal year. The objective is for Navy warships to go in and out of those ports. In addition, Navy anti-submarine and supply planes are expected to use upgraded airstrips at Mombasa.

Egypt's military facilities at Ras Banas on the Red Sea are shaping up as the closest thing to a launching pad for American military power, with $126 million earmarked for improvements this year alone. But Pentagon officials said there is no plan to turn Ras Banas into a giant warehouse. The idea instead is to stage one airborne division there, which means limited storage of ammunition and fuel, plus provision for fighter planes. The runways will be enlarged to accommodate B52 bombers and large cargo planes, which would bring in supplies after the troops went to the trouble spot.

Oman, which borders oil-rich Saudi Arabia and extends out into that oil seaway, the Persian Gulf, is slated to get $85 million this year in military improvements at its island of Masira. The Pentagon's plan is to base a small force of fighter planes at Masira, along with limited amounts of fuel and ammunition.

Diego Garcia, though about six steaming days by cargo ship from the Persian Gulf, is still considered by defense officials as the best place to store materiel for troops sent to that area. Eight cargo ships kept anchored in the lagoon off Diego Garcia carry enough tanks, water, food and ammunition to keep 11,000 Marines supplied for 30 days of battle. Marine officials said two inspections six months apart found that the gear in the dehumidified holds of those ships remained ready for use, strengthening policymakers' faith in storing weaponry afloat rather than on land.

Former Central Intelligence Agency director Stansfield Turner, a retired admiral, endorsed the emphasis of sea over land as launching pads into troubled areas. He told a breakfast meeting of reporters that there is "too much emphasis" on bases in the Middle East. To cover bets around the world, the United States should stress Navy and Marine forces, including ability to assault hostile beaches from the sea. He added that these forces must be able to count on quick resupply by air and sea. In distributing defense dollars, the admiral said, there has been "too much strategic-nuclear."