Ideas:

1. This is probably a picture of a person hanging upside down.

Normally we would then euphemistically call him 'fish' (ika), i.e. a victim bound up in a tree and soon to become offered to the Gods (and then potentially also made into food for the victorius).

But I doubt that this is the correct interpretation in this instance, in part because there is a sign included: The corpse is not hanging as it should, i.e. straight down. Instead it looks more like the bound up body was floating on the surface of the sea. However, it might be too imaginative to compare it to the coffin of Osiris in the Egyptian myth.

2. The unusual way in which it is hanging might be 'read' as a way to tell that it is not a real victim but a symbol. This symbol could for instance be a way to write 'tapu'. Women and children are not allowed near a hanging victim. And according to Barthel tapu = sacrifice.

In the modern Maori language Sunday is called Ratapu (and ra is 'day', 'sun, etc). The names of the other days of the week are borrowings from the English language. Perhaps Sunday was called Ratapu on Easter Island as well?