Books in the Marina the Mermaid series in order

Marina the mermaid explores a sunken ship with her catfish, Mr Stripe. There might be treasure, but also danger!

Marina the mermaid tries to get rid of a gigantic sea serpent, before it eats all the poor sea animals.

Pirates are attacking an innocent sailboat. Marina the mermaid tries to talk sense to the pirates, but in the end, she has to show them who is boss.