I don't really believe that this is the case. Just look at the size difference between Booskaboo in SD3 and the Turtle Island in SoM. Booskaboo is about as big as an elephant. Only three or four people can ride on it at a time (and actually fit on its shell). Whereas Turtle Island is large enough for an entire village to fit on top of it. Unless of course it grew that much larger than it originally was, but I doubt it. And the idea of Booskaboo's dead shell remains floating motionless in the middle of the ocean is not only disturbing and morbid, but also highly unlikely as a floating empty shell would be constantly moved by waves and could not be stable enough to have an entire village built on top of it. The villagers also say that "this island is actually a giant turtle" or something along those lines, but we don't know for sure if it's true or not. They could just think that because of its shape. Not only that, but said "gaint turtle" could also still be alive. That's just my opinion, but I could be wrong.