I was able to finally figure it out and finish, but it took a while. I explored the island several times, and went into the water looking for what I was missing to interact with. After falling off the cliff and landing on the shore I eventually found the corpse and realized what to do. The corpses blend into the terrain too much. What to do is not intuitive, but it is made harder by not being able to easily see the corpse at a reasonable distance. Even looking for them, and know roughly where to look, I sometimes had to circle around to find them, but that first one when not knowing what to look for was partly luck. I was getting ready to give up when I finally found it.