I love the first line--it is definitely engaging. I understand the second sentence, but the phrasing is a bit awkward.
Your subtle suggestion of what has happened to her at the end of the second paragraph is remarkable. The image is at once troubling and beautiful, and is another fantastic hook for the reader.
In the third paragraph, it is unusual to see such a beautiful image pertaining to such a repulsive bodily function. The start of that sentence ("I...arm") seems to be missing a word. I had to check back to see if I had skipped over a word while reading it.
The tension in the relationship between the two characters is well-written--you are excellent at the "show, don't tell" style of writing.
I am slightly confused about what, exactly happens at the ending, but I suppose it is intended to be ambiguous.
Your style reminds me, vaguely, of Jhumpa Lahiri, mostly because of the subtlety of the language and imagery of the ocean.
I really enjoyed this story! Nice work.