In an effort to save "boat people" fleeing Vietnam from perishing at sea, the Carter administration has decided to guarantee the resettlement of all such refugees picked up by ships under U.S. ownership or registration.

Administration sources said yesterday this policy decision had been worked out by the State Department and the Immigration and Naturalization Service to encourage U.S. ships to rescue refugees found in Southeast Asian waters.

Although no statistics are available on the number of refugees who have died during attempted escapes by sea, a large number are believed to have lost their lives though drowning, starvation and exposure. Many of the escapes are made in overloaded, unseaworthy or poorly provisioned small craft.

Since the flow of refugees from Vietnam began, U.S. policy has encouraged American ships to pick up "boat people" and take them to nearby ports. However, many ships have been inhibited from making rescues because of the reluctance of some Asian countries to admit the refugees.

In some cases, ships that have "boat people" aboard have been refused admittance to Far Eastern ports, even when they had cargos to unload, or have been kept under close guard with the crews not being allowed to disembark.

To overcome that problem, the sources said, the United States now will make known to Asian governments its willingness to arrange the speedy removal and relocation of any refugees brought into their countries' ports by American ships.

Some of these refugees will be brought to the United States, and efforts will be made to relocate others, particularly those with relatives already in other countries willing to receive them, the sources said. They added that it is not possible to tell at this point how many might end up in the United States.

According to State Department figures, approximately 5,800 "boat people" have landed in different Far Eastern countries during the last two months. Those nations with particularly large numbers are Thailand, Malaysia, Singapore, the Philippines, Indonesia, Taiwan, Japan, South Korea and Hong Kong.

Since last August, approximately 12,500 "boat people" have been resettled in the United States, and another 12,500 will be admitted under Attorney General Griffin B. Bell's emergency "parole authority" over the coming year.

In the past three years, the United States has admitted 164,000 Indochinese refugees.