LONDON -- The London-based maritime insurance market is waiting with skeptical interest for the Reagan administration to begin implementing its plans to protect Persian Gulf shipping with American flags and warships.

If the U.S. presence succeeds in stemming attacks by both Iranian and Iraqi forces, some brokers and underwriters here believe, the surcharges levied for insuring oil tankers and other vessels in the gulf may eventually decrease.

Others, however, believe that the success of the American plan may only shift the attention of the combatants to other insured targets. "It's not going to reduce the aggregate amount of claims, although it may well change the way in which losses occur," said Stephen Merrett, whose underwriting syndicate at Lloyd's of London was, until recently, one of the leading insurers of gulf shipping.

An increase in sabotage and terrorism is likely, Merrett said, against individuals and refineries viewed as "softer targets than the escorted vessels." At the same time, the risk of "unattributable hits," particularly from undersea mines, is expected to grow.

Until it "proves it will reduce the risk," said a Lloyd's broker, the administration plan "doesn't make any difference in insurance terms . . . . I suspect the underwriters will just sit tight, charge the same rates and react according to what happens."

At the top of the high-risk market is the shuttle carrying crude oil from Kharg Island and other Iranian terminals at the top of the gulf for loading into tankers hovering at the Strait of Hormuz, theoretically outside Iraqi aircraft range. The shuttle is "virtually uninsurable" now, said one broker. For a price, markets can still be found here to cover a one-time trip to Kharg. But no one, he said, is interested in insuring repeated trips along a corridor open to near-certain Iraqi air attack.

For the few foreign vessels now participating in the shuttle, Iran has set up its own insurance company, promising to pay off losses with Iranian bank-guaranteed policies. According to brokers here, however, none of the several claims already lodged in the system has yet been paid.

Last spring, ships traveling to and from Kuwait, a particular Iranian target in recent months, were briefly subject to a higher rate surcharge than the rest of the insurable gulf waters, sources here said. But aside from the Iranian shuttle, they said, the rates now are relatively stable, reflecting a lack of activity on either side in recent weeks.

Some insurers, like the Merrett Syndicate, have dropped out of the gulf altogether because of the high risk and what they describe as the confused nature of doing business there. The market is highly competitive and innovative, with premiums often parceled out through brokers among an array of "reinsurers," who essentially insure the primary policy-holder against catastrophic loss above a certain sum.

"The perception is that they are making money," said Merrett. "They may indeed, but whether they're making enough to justify all the risks they're taking is perhaps another question."

Merrett's conservative view, however, remains a minority one in the rush to pick up gulf premiums. "One or two have gotten out," said a broker specializing in the region. "But there are plenty of markets. There is no problem getting cover."

Despite the recent increase in U.S. concern, analysts said the assessed risk of sailing through the gulf has remained fairly constant since last year, when it rose abruptly as the combatants changed tactics to regular sea attacks.

According to the latest report by Lloyd's Intelligence Department, 53 vessels, including the USS Stark, have been attacked or damaged in the first 5 1/2 months of this year.

That figure compares to a total of 107 attacks in 1986, by far the greatest number since the war began in 1981. Between 1981 and 1986, 226 known deaths resulted from attacks in the gulf, compared to 47 so far this year, 37 of them aboard the Stark.

Who gets hit, when and where in the gulf fluctuates, said Lloyd's casualty reporting officer Roger Lowes, with trends coming and going every several months. The number of attacks over the years has been fairly evenly divided between Iran and Iraq, Lowes said.

This year, he said, there has been a tendency by Iran to attack vessels heading to or from Kuwait, "the reason being obviously that they say Kuwait is trading with Iraq." While Iraq specializes in air attacks, aircraft-short Iran has relied increasingly on attacks launched from small patrol boats and helicopters firing relatively light weapons.

In general, the Lloyd's broker said, "The Iraqis tend to do a lot more damage than the Iranians, unless the Iranians hit a conventional vessel rather than a tanker." Compared to an air-launched Iraqi Exocet missile, he said "a couple of antitank missiles fired from a helicopter" can do relatively little harm.

Even an Exocet, however, usually presents a relatively minor threat to the huge oil tankers. Unlike the Stark, a guided missile frigate, the broker said, "a lot of a tanker is empty space or full of crude oil."

But maritime insurance analysts still consider the gulf by far the riskiest piece of navigable water on the globe. Rates there remain the highest in the world.

Paid insurance claims for ships damaged since the war began total approximately $1.5 billion, according to several sources here. Although that figure includes 110 vessels that have been "paid out" as total losses, all but 17 of them were cargo vessels trapped and abandoned in the estuaries of the upper reaches of the gulf when the war began.

Reflecting the highly secretive market here, the sources were reluctant to estimate the total amount of premium that has been paid.

Most ocean-going vessels carry both standard and special war risk insurance. But war insurance, ironically, specifically excludes those areas where conflict is thought likely or is actually in progress. In addition to the gulf nations, Israel, Lebanon, Syria, Libya and Angola comprise the current international list of excluded countries.

Any ship entering the waters of these countries is obliged to inform its insurer, who then charges an additional premium that fluctuates according to the perceived threat in that area at the time. The extra insurance, limited to a period of seven to 14 days, can cost as little as 0.25 percent of total hull value -- the current basic rate for the gulf -- or as much as double the normal premium for a trip to Kharg Island.