The Mutation (Animorphs #36)

In this thrilling installment, the Animorphs and Ax confront a chilling discovery from free Hork-Bajir on Earth: Visser Three’s attempt to create amphibious Hork-Bajir and his obsession with a sunken Pemalite ship. This leads to the construction of the advanced underwater craft, SeaBlade, which poses a significant threat as it seeks the ship's technology. After transforming into orcas to engage SeaBlade, they stumble upon an extraordinary underwater civilization known as the Nartecs, who possess a unique blend of human traits. Now, the Animorphs and Ax must not only navigate this new conflict with the Nartecs but also collaborate with Visser Three to thwart a greater danger.