A turtle overall resembles its base-game counterpart with the slight exception of its texture being brighter and brighter green stripes along its shell.

Turtles tend to swim around aimlessly. Unlike base game turtles, they have a unique idle behavior where they lie on their shells and float motionless, like relaxing. They are also capable of doing a flip, as seen in the caged turtles in the Radiant Ravine. In Radiant Ravine, turtles act as non-attackable mobs needing to be rescued.

https://minecraft.fandom.com/wiki/Minecraft_Dungeons:Turtle