Books like Mermaid Party (Not Quite a Mermaid #4) by Linda Chapman

Electra, the nine-year old mermaid with legs instead of a tail is back and in search of even more adventure! It's New Year on Mermaid Island. Every year a young Queen of the New Year is chosen and Electra is desperate for it to be her! So she might get chosen, Electra and her dolphin friend, Splash, decide to swim over to the grotto to help the grown-ups decorate it for the New Party. But Electra and Splash get carried away and make a BIG mess in the grotto. Sent away to behave, the two of them discover tunnels at the back of the grotto which lead to the deep sea. All the tunnels are safely blocked up except one. Electra and Splash try and tell the adults but they are too busy fixing the mess to listen! So when a group of sharks swim down the tunnel and threaten to eat everybody it's up to Electra and Splash (and a giant ink squirting squid) to save the day!

First publish date: 2006