I loved the story, Dann, as you know, and I definitely thought she was a mermaid! I have to confess I didn't understand the relevance of the title, so I imagine I did miss something you intended regarding the narrator and who he/she was, but even after several re-reads I'm still none the wiser. Unless it's to do with the bit in the last line about wandering "there"? I assumed that the "there" was wherever s/he found the shell, and since you hadn't told us where that was I marked the story down slightly, but is it the shell itself??
A couple of other tiny things. In the line "She said she was "new" and that's why she was so small" I read the "that's why" as her explanation, and so strictly that should be in past tense also ("that's" = that is, ie present) but on examining it again now I suppose it's arguable it's not her explanation but his deduction from what she said, so present tense might just be OK (and it didn't affect how I decided to vote). The other thing was "a lisp of a wave" -- is that a USism? I only know "lisp" as a minor speech impediment and I was confused how that related to a wave -- on the one hand I quite liked the feel of the expression, but it didn't stand up to examination when I tried to work it out, so I was rather torn about it, and because I kind of mentally stuttered with it I was taken out of the story a bit.
But a good story as far as I was concerned, and I loved the image of her in the shell like Botticelli's Venus, only with a tail!