MAJURO, Marshall Islands (AFP) - Pacific castaway Jose Salvador Alvarenga was preparing on Wednesday for a faster and more comfortable journey home after 13 months adrift in a small fishing boat, as his mother called his rescue "a divine miracle".

Officials in the Marshall Islands said Mr Alvarenga would depart the tiny Pacific nation on Friday for Hawaii, before travelling on to El Salvador or Mexico.

"Most likely he will be repatriated back to El Salvador," foreign affairs official Anjanette Kattil told AFP.

"Initially we thought he was Mexican but even if he was not, the Marshall Islands would render assistance to anyone who washes up on our shores."

Mr Alvarenga was born in El Salvador but had lived for years in Mexico, where he says he set off on a fishing trip in late 2012 before becoming lost and drifting some 12,500-kilometres to the Marshalls in a small boat.

The 37-year-old said he survived by eating raw fish and birds as well as drinking turtle blood, urine and rainwater for 13 months, but a teenage companion named Xiguel starved to death during the ordeal.

His family, including a 14-year-old daughter named Fatima who has no recollection of the father who left for Mexico before she was born, said they were looking forward to welcoming him home.

"It is a divine miracle, a sign that God was compassionate with our son's life," his mother Maria Julia said from her home on El Salvador's Pacific coast, tears of joy streaming down her face.

"I kept thinking that one day he would come back to us, that God wants him to return to our house."

Mr Alvarenga was released from hospital in the Marshalls capital Majuro late Tuesday after a medical check-up found he was suffering severe dehydration and the effect of a poor diet but otherwise healthy.

After shaving the bushy beard that grew during his months adrift, he was keeping a low profile in a local hotel on Wednesday, with two policemen guarding his room.

He has become something of a celebrity in the Marshall Islands and Kattil said locals had taken him to their hearts.

"People have anonymously arrived at hospital to bring him food, clothes, toiletries and blankets," she said.

He is expected to hold a press conference before he leaves the Marshall Islands.

The stockily-built fisherman appeared remarkably healthy just days after being found wandering disorientated and clad only in ragged underpants on the coral atoll where his boat washed up.

There were some contradictions regarding dates in various media interviews he has given since being rescued, but authorities in Mexico confirmed they launched an air and sea search for him and a missing boy in November 2012.

Fishermen in Mexico's southern Chiapas state also said they remembered Mr Alvarenga, who was known by the nickname "La Chanca" or "fatty".

"We are surprised, but there is no doubt it's him," said fisherman William Uscanga after seeing a picture of the castaway.

While some have expressed scepticism about Mr Alvarenga's account, survival experts such as Mr Hilmar Snorrason of Britain's International Association for Safety and Survival Training said the remarkable feat was - in theory - possible.

"I can imagine there are many people who think this story is unbelievable. (But) theoretically... my answer is yes, it is possible," he told AFP.

"He should be given the benefit of the doubt."

El Salvador does not have diplomatic relations with the Marshall Islands but Mexico does, and a Mexican official was due in Majuro late Wednesday to coordinate Mr Alvarenga's repatriation.

In an interview with AFP from his hospital on Tuesday, Mr Alvarenga said he had suicidal thoughts during his trip but was sustained by dreams of reuniting with his family and eating tortilla and chicken.

His mother was eager to oblige.

"We will make him a big meal, but we won't feed him fish because he must be bored of eating that," she said. "We will make him a big plate of meat, beans and cheese to help him recover."