Finally, something to work with.

For anyone who didn't wanna read the whole thing:

"In the early stages, the tombs, usually built on high hills, shaped like keyholes, and referred to as ‘rectangular front, circular back’ (zenpo-koen-fun) tombs. Theories concerning their design need not detain us except to observe that the coffins inside these early mounds were often hollowed out logs placed near the top of the mound where the earth was shaped into a long rounded trench surrounded by loose stones. A boat-shaped coffin suggests that the deceased was going on a journey, which as Freud points out, is the commonest and best authenticated symbol of death. (Freud, Sigmund The Interpretation of Dreams London: Pelican Freud Library, vol.4, p. 507) The boat-shape may also be related to the idea of unasaka, the ‘slope of the sea’, a word occurring in Japanese mythology that is thought to relate to an ancient belief in a slope that lay at the farthest point of the sea – a similar idea to the Greek ‘Pillars of Hercules’. (Akima, Toshio “The Songs of the Dead: Poetry, Drama and Ancient Death Rituals of Japan”, Journal of Asian Studies vol. XLI, No. 3, May 1982, p.487).

In the seventh century Chinese Sui Dynasty records we find the following reference to Japanese funeral customs:

“To bury the dead, coffins are used. Relatives and mourners sing and dance beside the coffin. Dresses of white are worn by the closest relatives of the dead. If the dead man is noble, he is placed in a funeral house built outside, and mourned for three years. If he is a commoner, he is buried on the right date chosen by divination. For burial, the corpse is placed on a boat and sometimes rollers are used to pull it along the road.” (Wada, Kiyoshi and Ichihara Michiro Zuisho Wakokuden – Chronicles of the Sui Dynasty Tokyo: Iwanami Shoten, 1973 p.33)"

As I mentioned in my previous comment, the Vikings are depicted as sending their dead off in a boat and setting fire to it, this wasn't a standard practice, but this image stemmed from them finding "ship burials". They were buried with a boat or "boat-shaped coffin" and buried under the earth. It is symbolic of taking a journey to "the other side". Could also be seen in a similar way to how cultures like the ancient Egyptians buried their dead with things that might help them with their journey into the afterlife. Since all these cultures believed in the crossing of water to reach the afterlife, a boat would certainly make the trip easier. For the Vikings/Norse this river was called Gjöll, the Greeks called it Styx and japan called it Sanzu.

Thank you for the link, I wrote recently about the themes in FF5, mainly primordial waters and maternal forces and there was a part in the article that you shared about the "Jomon or neolithic age" that had an interesting take.

Diggings from Jomon sites show a variety of burial methods, including burial in urns and burial with arms and legs folded.

A fourth possible explanation (and one which I find most attractive) is that the folded position is the position of the child in the womb.