Tamed you'll be able to build on its back. When diving he provides an oxygen bubble on his back allowing you to submerge your base on the floor of the ocean.
Are you turtley enough for the turtle club?

This boi will be able to be your mobile water base while looking cool af!

It provides oxygen for anything on its back

Rock nodes and plants spawn on top of his flattish shell. Provides shell-wide air bubble when submerged. Is massive, and able to be built upon. Looks super cool.