The Ocean Squid Explorers' Club : The Polar Bear Explorers' Club, Book 4(Polar Bear Explorers' Club‪)‬

In this fourth explorers adventure we meet Ursula, a part mermaid girl who is determined to follow in Stella's footsteps and help her to defeat the Collector!
Ursula knew the submarine wasn't designed to go this deep, but what choice did they have? No one wanted to get swallowed up by a colossal sneezing jellyfish!
In a distant watery corner of the Explorers Kingdom, a submarine engineer, Ursula, is determined to become an explorer. Unfortunately, she hides an extraordinary secret, which makes her the sworn enemy of the Ocean Squid Explorers' Club were they ever to find out . . . But when The Collector threatens the Club, Ursula throws caution to the wind and leads an expedition through treacherous waters, filled with gremlins, aboard the Blowfish submarine - and joined by her friends Max, Genie and Jai - and even her idol Stella!
Thrilling, heart-pounding adventure with exquisite detail throughout. Alex Bell's world-building is second to none.
Praise for the series: “A fantastic frosty adventure.” SUNDAY EXPRESS