Linda Chapman 
Not Quite a Mermaid: Mermaid Promise 

Electra has had a great idea. It’s been a year since her best dolphin friend, Splash, came to live on Mermaid Island so she’s going to throw him a party! But when Splash meets some other wild dolphins, Electra is scared that he will want to swim away with them for ever. How can Electra make Splash stay?