White Mermaid was a spirit of a mermaid living inside a ship's figurehead in the animated H2O: Mermaid Adventures.
Edward the Cruel was a fearsome pirate who owned a ship called "The White Mermaid", the pirate could not be defeated because White Mermaid appeared in Edward's dreams and warned him of danger. However, Edward's success as a pirate attracted other pirates, who were after his gold. Eventually even the mermaid spirit wasn't able to help him, and he sank his ship, so his gold would be safe. White mermaid guarded his gold eternally.
Bernie found the figurehead, and notified of it to Emma, Cleo, and Rikki. The girls hoisted it to surface, to Lewis McCartney, and then took it to his garage. After the girls shard a warning dream from the White Mermaid, they located the wreck of the pirate ship and took the statue back to it.
When Stanley Finch returned to Dolphin City to look for the wreck of the White Mermaid pirate ship, White Mermaid again warned the three mermaid girls. With their combined efforts they were able to lead the archeologist away from the wreck. After a little mishap with a Stone Circle, the girls used the full moon power, and the fish friends to move the wreck of the white mermaid.
When Rikki temporarily lost her ability to transform back to human, White Mermaid revealed Rikki the location of a secret room in the the pirate ship, where she could spend her nights.