Mermaid Snow

Mermaid Snow is the femslash ship between Snow White and Ariel from the Once Upon A Time and Disney fandoms.

Canon
Snow White meets Ariel during a time when she was countlessly running away from the Evil Queen, where they become fast friends; shortly after Ariel saves Snow from drowning.

Years later in the modern events in "The Jolly Roger", Ariel asks Mary Margaret (Snow White's curse name) to help her find Eric in Storybrooke, to which she agrees since they are friends and Ariel had once helped Snow in a time of need.

Once Upon A Time

 * Ariel choses Snow over her chance to remain human, so she could be with Eric before he leaves, in order to save her from the Evil Queen.

Fanon
Because of the friendship that they formed in the Once Upon A Time series and are two Disney owned and based characters that are Disney Princesses who have been femslash in Disney crossover ships that focuses on its princesses, there are fans who use the ship and some of its titles in both fandoms.

Since Ralph Breaks the Interest is a canon Disney crossover that has a version of the two know each other, like their once counterparts do in the Once Upon A Time series, fans of the Disney side of the ship sometimes have Snow White and Ariel in the modern, style clothes that the netizen versions of them wear in the film.

On AO3, the OUaT side of the ship has 4 fanfics.

Fandom
FAN FICTION
 * Ariel/Snow White (Sofia the First) on FanFiction.Net
 * Ariel/Snow White (Kingdom Hearts) on FanFiction.Net
 * Ariel/Mary Blanchard (Once Upon A Time) on FanFiction.Net
 * Ariel/Mary Blanchard (Once Upon A Time) on FanFiction.Net

TUMBLR

Trivia

 * In the Disney film, Ralph Breaks the Internet, Snow White and Ariel appear as online versions of themselves known as "netizens", the inhabitants of the digital made world that lies within the real-life internet. Where they are two of the fourteen Disney Princesses that are from the expo-like Disney website, Oh My Disney.
 * They are two of the netizen princesses (and queen) that join in the "Neither do we!" with the rest of the motherless princesses.
 * They are two of the Disney Princess characters to appear in the first game of the Kingdom Hearts series.
 * As well as Ariel making her second reappearance one in Kingdom Hearts II and her third one as a summon character in Kingdom Hearts III, along a memory version of Ariel being seen in Kingdom Hearts: Chain of Memories. While Snow White makes her own reappearance in one of the prequel games of the series, Kingdom Hearts: Birth by Sleep.
 * Both Snow White and Ariel appear in the Disney Junior series show, Sofia the First, when the Amulet of Avalor summoned them to aid Sofia.
 * They are two of the limit time characters in Disney Magic Kingdoms.