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Hi Rosey, here I am on the final part.

I felt my lungs suddenly expand, as if a laugh had died.

? I don't quite understand this imagery.

I didn’t say what was on my mind though. I couldn’t make such a choice so quickly. Not with the memories I had.

It's fine to say this, but even I don't know what is on the MC's mind. It just feels rather cryptic. Use first person to your advantage--let the reader really get into the MC's head. Right now, I'm not sure exactly why she's feeling the way she is. In fact, I've been feeling oddly distant from her throughout the story.

There was something in his hand, what looked like a shell attached to some seaweed.

"That" instead of "what", and the comma is unnecessary.

We were out of view enough, and the water would be shallow enough I could just blame wet sandals on having walked in the tide.

Icky repetition of "enough", and "out of view enough" is too vague a phrasing. I'm not quite sure what that's supposed to mean.

I was about to snap that I wasn’t worried about him hurting me, when I realized I was. The risk of him hurting me from impatience was growing by the second

'

I don't understand why Carl would want to hurt the MC. You haven't really given any indication that he's angry, or he's the type to hurt the girl he loves.

The water rose to near my knee

Better phrased as "the water rose to my knees", since she has two knees (I should hope), and "near" is vague fluff, what my English teacher would call "lard".

“You’re alright?”

It's more standard to say "You all right?" (at least where I live), and "all right" is actually two separate words.

I had seen this kind of clasp before, but never in such a marital.

Do you mean "material"?

Carl put his hand on my neck to keep my head still and kissed my cheek.

Hold on, I thought Carl couldn't kiss her, or she'd turn into a mermaid or someting? Unless I'm just being ignorant...or does that only apply to an open-mouth kiss?

went right to the jagged rocks where Carl had said goodbye to me ten months before.

I've noticed this a lot, but you really like this word "went". I don't. =/ It's not specific enough; every "went" can be an "ambled", "dashed", "strolled", "stumbled", etcetera. The stronger the verb you use, the stronger the mental image you paint in the reader's mind will be.

So here my story ends. Not happily, not unhappily. That summer just became what was. And ever after, I always looked to the sea, holding a seaweed and shell necklace in my hand, hoping it would glow.

A very effective ending. It's emotional and understated at the same time, and I like how you've ended the story not on a uniformly-happy or uniformly-sad note, but in a bittersweet way. There's a shade of ambiguity to this ending that makes it work, especially as a subtle rejection of standard fairytale tropes. You've actually done a very good job of building up ambiguity and not-so-clear-cut conflicts throughout this story; I've never really noticed it until this ending, though.

Overall thoughts:

All right...I think you know already that I believe you capable of producing a much better work. This ending worked much better for me than the first and second parts did, though. Especially the last bit, when Alexandra is reflecting on how her life went after Carl--it had a level of emotion to it that the rest of the story was lacking in. For the first time, I was truly feeling something for Alexandra. The ending contains a hint of what the story might be if the emotion is stronger: suffused with a sense of loss, at the same time with a sense of moving forward. It's my favorite kind of ending, so maybe I'm just biased, but I think if you increase the emotional atmosphere of the rest of the work, it'd be a much more effective story.

Like I've said before, a lot of the problems with this piece stem from not really feeling any emotion. In this case, there's more emotion, but a lot of that is conflicting and confusing. I couldn't follow why Alexandra was afraid that Carl would hurt her, and I still am not entirely sure what it is she feels about Carl. For some reason, I don't understand why she would feel so betrayed. Well, I suppose it'd be a shock if the guy I had a crush on turned out to be a merman (ugh, must resist urge to make a Zoolander joke), but I don't exactly follow on where this sense of betrayal came from.

Reading this part, I realized that much of the, well, relative lack of emotion comes from vagueness in language. This is something I noticed in Cat Steps and Blood as well--you tend to use very basic language such as that ubiquitous "went" and "talking" and "walking" and "thing" and such instead of stronger, more specific verbs. Not only does it make the prose somewhat less engaging (though your style overall is very clear and conveys the point you want to make very effectively), but it makes it harder for the reader to connect with your character. If you use strong verbs with strong connotations, that will give us a better view into Alexandra's thoughts. Take as an example the descriptions of Carl: they were all perfectly neutral. There was no indication whether she was afraid, was disgusted, or reluctantly thought he had a kind of beauty to him. You just describe him as having a slate blue and that's that. Describing the water glinting off his scales as "like a slimy fish" would convey disgust; on the other hand, describing it "like morning dew" would imply that she finds him beautiful. Keep that in mind.

This story does have the potential to be a great one, and I love the ending, but right now it's a bit hampered by the above issues. Keep on working, though. I'll be more than happy to take a look at an edited version as well. ^^

PM if you have any questions, and good luck in the contest!