Theory about the turtle...
As others have pointed out, I believe that the red turtle transforming into a woman and them having a child and all that was just an hallucination. There are some clues pointing to this throughout the beginning of the movie:
1) We know the man is already prone to hallucinations as he imagines the musicians playing on the shore. I believe this is shown to establish the fact that the man is not a reliable observer anymore and that not everything we see is real from this point on.
2) When the man finds the dead seal, there are some flies circling it. Then the man cuts it open and realizes it's rotten. I think this was shown to make the statement that flies = dead. Later on, when the turtle is lying on its carapace and apparently dead, we see flies flying around it. I believe this was shown to tell us that the turtle was, in fact, dead, and everything else that we see afterwards with the woman and the child are hallucinations fueled by loneliness and remorse for killing the turtle.