Something that wasn't explained?

Just watched it. Agree. I thought they were going someplace with it. Stories can leave hints, allow for I inferences, not explain everything, but they shouldn't require the viewer to speculate. Here my guess would be they killed a raft mate to make supplies last longer, but perhaps there wasn't a more benign explanation. It probably would have been better left out.

I appreciated them avoiding casting the boyfriend as an archetypical coward, first he scorns her experience, then shies away when she needs him the most. But he showed great bravery in both encounters. He might have survived had he just let the monster take her.

The movie never explains the blood, but my take was the boyfriend - and possibly the other girl with him - killed him because they were running out of water. There was also a lot of blood in the raft. The boyfriend wasn’t a coward, but he was definitely all about him.