Sea otters are represented by bone and teeth in the Ozette deposits. Particularly interesting is a wooden box with bone tipped arrows of a type used for sea otters in historic times. Only canoe hunting would be practical for such a heavy quiver.
Other sea mammals had to be harpooned to prevent escape, but sea otters could be successfully shot with barbed arrows. These animals have little blubber, so arrows could penetrate enough to hold firm even without the heavy thrust of a harpoon. Furthermore, when struck, sea otters didn't dive deep or swim far. They stayed within the range of additional shots.
No animals anywhere have richer fur.