Sporting a bushy beard and clutching a can of Coke, a castaway who says he survived more than a year adrift in the Pacific Ocean arrived in the Marshall Islands capital Majuro on Monday.

The castaway told US ambassador Thomas Armbruster, who was acting as an interpreter for Marshall Islands authorities, that he was originally from El Salvador but had been living in Mexico for 15 years before his epic voyage.

"He said he is a shrimp and shark fisherman," Armbruster said on Monday in Majuro minutes after talking to him. "He looked better than one would expect."

And foreign ministry officials said he had told them during a debriefing aboard the patrol boat that his full name was Jose Salvador Albarengo.

He was found disorientated and clad only in ragged underpants last Thursday, after his 24-foot (7.3-metre) fibreglass boat floated onto a reef at Ebon Atoll, the southernmost cluster of coral islands in the Marshalls.

Unable to speak English, he communicated to his rescuers through pictures and gestures that he had survived by eating turtles, birds and fish and drinking turtle blood when there was no rain.

No details have yet emerged about why he began drifting the 12,500 kilometre (8,000 mile) expanse between southern Mexico and the Marshall Islands, or about the fate of a companion he said had died a few months ago.

Marshall Islands immigration chief Damien Jacklick said authorities were still gathering information and the foreign affairs department planned to contact overseas officials for his repatriation.

"With the help of the US ambassador, we were able to obtain information on his family members in El Salvador and the United States," he said. "We hope this information will help us track down his family."

Medics plan to give Albarengo a thorough medical check before he is interviewed by detectives.