Pod People (http://archiveofourown.org/series/29131)

Here is one:

Saving You, Saving Me (http://virtualpersonal.livejournal.com/264034.html) by virtualpersonal
Summary: The first time Dean lays eyes on the merman imprisoned and on display in an aquarium at a seedy night club, he can't help being drawn to him. Though they can't communicate, a bond is formed between them and he finds it harder and harder to leave Sam in the hands of the merciless bastards using him.

This story is Wincest.

Well, spn_littlebro has a prompting theme (http://spn-littlebro.livejournal.com/11919.html) that allows you to leave prompts whenever you want, and the new community write_on69 is for posting and accepting J2 and Wincest fic challenges.

Thanks :)

Here There Be Monsters (http://bewaretheides15.livejournal.com/94302.html) by bewaretheides15

Hey there. I have this one fic that I absolutely love with a baby merman Sam.
Title: Something Fishy
Author: nanoks

The summary says it's Sam/Dean, but I really didn't think so when I read it. I think the warning is up because Dean goes to be holding baby merman Sam, and wakes up under big nonmerman Sam. Sam's clothes did not survive this transformation. (Shrugs)
If that's an objection for you you might want to avoid this one.

Summary: ‘Sam had been a baby merman for six hours and still had another six left to go’

I personally especially like the fact that this fic has pictures (really adorable pictures) of baby merman!Sammy
http://nanoks.livejournal.com/93864.html

http://www.fanfiction.net/s/6284407/1/Red-Fish (http://www.fanfiction.net/s/6284407/1/Red-Fish) is another take on the previous posters baby merman story

and http://www.fanfiction.net/s/7476374/1/Sam-s-Fish-Tale (http://www.fanfiction.net/s/7476374/1/Sam-s-Fish-Tale) is another multi-chapter merman Sam story.

Not a merman!Dean, but a nix: http://zannes.livejournal.com/21571.html

With a lot of hurt!goldfish.