A man who says he spent more than a year adrift in the Pacific ocean after a shark-fishing day-trip went disastrously wrong has described his ordeal for the first time, saying that he considered suicide after his 15-year-old shipmate died.

The man, named as 37-year-old José Salvador Alvarenga, said he set out from Mexico in his 23ft (seven-metre) fibreglass boat for a day's fishing on 21 December 2012 with a youth named Ezekiel, but was blown off course by a storm.

Thirteen months later he was washed ashore on the Ebon atoll, part of the Marshall Islands, having drifted 5,500 miles across the Pacific. "I didn't know the hour, nor the day, nor the date," he told the Telegraph at a hospital in Majaro, the capital of the Marshall Islands.

"I only knew the sun and the night … I never saw land. Pure ocean, pure ocean. It was very placid – only two days with big waves," he said.

Alvarenga told the Telegraph he wanted to take his own life when Ezekiel died four months into the voyage after refusing to eat. "For four days I wanted to kill myself. But I couldn't feel the desire – I didn't want to feel the pain. I couldn't do it.

"I had my mind on God. If I was going to die, I would be with God. So I wasn't scared ... I imagine this is an incredible story for people."

Alvarenga has said that he survived by catching seabirds and fish, and drinking turtle blood, rainwater – and at times his own urine.

Officials on the Marshall Islands have reacted cautiously to Alvarenga's story, which contains several discrepancies - possibly because there were no Spanish speakers on Ebon when he first arrived.

Speaking to the Telegraph, Alvarenga – who said he was originally from El Salvador – described his relief on sighting land.

"I had just killed a bird to eat and saw some trees. I cried, 'Oh God'," he said. "I got to land and had a mountain of sleep. In the morning I woke up and heard a rooster and saw chickens and saw a small house. I saw two native women screaming and yelling. I didn't have any clothes – I was only in my underwear and they were ripped and torn."

Alvarenga was taken to the capital, Majuro, on Monday, where he briefly met the US ambassador and other officials before being taken to a local hospital.

"It's hard for me to imagine someone surviving 13 months at sea," said the ambassador, Tom Armbruster, in Majuro. "But it's also hard to imagine how someone might arrive on Ebon out of the blue. Certainly this guy has had an ordeal, and has been at sea for some time."

Armbruster said the soft-spoken man seemed in reasonable health, and, rather than appearing emaciated, he looked puffy in places, including around his ankles. The ambassador said Alvarenga complained of pains in his joints and had a limp but was able to walk.

The ambassador, who speaks Spanish, said Alvarenga had lived in Mexico for 15 years and worked as a fisherman. "He talked about scooping up little fish that swam alongside the boat and eating them raw. He also said he ate birds, and drank birds' blood," he said.

"He thanked God, initially, that he had survived," the ambassador said. "He's very anxious to get back in touch with his employer, and also with the family of Ezekiel. That's his driving motivation at the moment."

However, Gee Bing, the acting secretary of foreign affairs for the Marshall Islands, said he was somewhat sceptical of Alvarenga's account after meeting him.

"It does sound like an incredible story and I'm not sure if I believe his story," Bing said. "When we saw him, he was not really thin compared to other survivors in the past. I may have some doubts. Once we start communicating with where he's from, we'll be able to find out more information."

Bing said the man had no identification with him and other details of his story remained sketchy, including the exact location of his departure from Mexico.

The man's health appeared to be good, but his blood pressure was a bit low, Bing said. After doctors give him the all-clear officials hope to repatriate him to Mexico – or whichever country is appropriate.

Jack Niedenthal, a film-maker based on Majuro said: "He got off the boat with a very bushy beard."

Niedenthal told Reuters after speaking briefly to Alvarenga through an interpreter: "He's having trouble walking, his legs are very skinny. I'm not ready to call this a hoax, I think this guy has done some serious time at sea."

Erik van Sebille, a Sydney-based oceanographer at the University of New South Wales, said there was a good chance that a boat drifting off Mexico's west coast would eventually be carried by currents to the Marshall Islands. He said such a journey would typically take 18 months to two years depending on the winds and currents, although 13 months was possible.

"The way that the currents in the Pacific work is that there is a very strong westerly current just north of the equator and that basically drives you directly from Mexico all the way toward Indonesia and in the path, you go right over the Marshall Islands," he said.