Honestly, this was kind of scary... for me I just didn't like how she was so enthralled by the sea that she either drowned herself or got swept away by the sea... at first when they were crying, I was like maybe she went overseas or sum, but then mari was old.. and they were in school uniforms and hmmm yea..
From what i understand...i dont think Kanan drown herself. It just that she didnt return after diving one day. And whatever Mari remembers during the few last pages was her memories recalling Kanan kissing her with that voices in her mind while looking at the sea. Then her soul found Kanan again, meaning to say Mari reached that old age and her time has come, reuniting with her long gone Kanan who has been wandering by the sea.
This is one of the most beautiful things I've ever read. I don't understand why everyone finds it so confusing? Kanan went on a dive, drowned, and when Mari died Kanan came to get her. Simple. They went back to being young (the afterlife would kind of suck if you were old in it- unless you wanted to be)- I think Mari went back to when she was happiest and that was when she was in high school with Kanan. Then their souls go off together, reunited at last.