Leaden winter skies lour on a coastguard boat as it cuts its way through the seas off Japan's tsunami-wrecked shores.

Somewhere under these unforgiving grey waters lie hundreds, perhaps thousands of bodies; the unfound, unclaimed dead of one of the country's worst ever disasters.

Even though the hunt on these sullen seas goes on every day, Yoshifumi Suzuki says none of his coastguard colleagues has seen a single corpse since the partial remains of a man were untangled from a fishing net in November.

But they are not prepared to give up.

"If we don't do this, nobody will," Suzuki said.

"We want to continue the search until we find the very last one. I want to return people to their families not because it is my official duty, but because it is my duty as a human being."

"The (missing) person is in the mind of his or her family but they still want proof that the person lived in this world. I think it's hard for them to accept the reality" without a body, he said.

The massive tsunami that pummelled Japan in March last year claimed more than 19,000 lives. One-in-six of the dead has never been found.

In the ravaged port city of Ishinomaki and the adjacent town of Onagawa, 20 percent of the 4,700 victims are still officially listed as missing, although no-one believes they will be found alive.

"If the sea was transparent, we would be able to find more people. It's awful," Suzuki told AFP aboard the 26-ton Shimakaze.

Occasional snow stings the five-strong crew as they peer through winter fog at the surface of the sea and use underwater sonar equipment to detect submerged cars or other debris that can trap bodies.

If they see anything that suggests a corpse, they call for divers.

But visibility below the surface is only one metre (three feet) and the cold temperature means frogmen can be in the water for no more than a few minutes.

Yoshiyuki Kikuchi, the captain of the Shimakaze, remembers the horror of the tsunami and the frantic days he spent in the initial search for bodies.

When he got the tsunami warning, Kikuchi sailed out to sea to protect his boat -- waves are smaller offshore and easier to ride over.

He saw a huge wall of water coming his way 10 kilometres (six miles) offshore.

"It was a large swell that I had never seen in this area before," he said.

"Then I began to see rubble coming (from the shore) on the backwash -- destroyed houses, tyres and cargo containers -- it was a horrific scene."

His passage blocked by the swirling clouds of flotsam, Kikuchi was only able to return to port on the third day after the disaster and immediately began the task of recovering bodies.

It is a task he has done over the more than 10 months since the tragedy struck, and one that is getting harder all the time.

"We are doing our best but it is becoming more and more difficult," Kikuchi said.

On land, a police search continues through Kesennuma, a city practically wiped out by the tsunami and subsequent fires, but no bodies have been unearthed since December.

For parents of children at the destroyed Okawa elementary school, there can be no end until the last four of the 74 dead youngsters are found.

Wrapped up against the bitter January cold, four men dig through the mud and soil in their sad hunt for remains.

"Children are still missing. We are here so that they can go home as soon as possible," said one of the men, who did not want to give his name.

Two policemen on secondment from Tokyo stopped at the school to offer prayers at an altar built to the memory of the 74 children and 10 school officials killed on March 11.

"My heart aches," said one. "I've seen parents who are looking for their children, including a mother who even obtained a licence to operate heavy equipment for the search.

"I think their sorrow will never leave their hearts... but I sincerely hope the missing people will be found soon."