We love turtle worlds. We already have a question about turtle planets : How do I prevent my turtle from collapsing under its own gravity? but let's scale it down.
I would like to have a waterball planet inhabited by gigantic oceanic creatures. The biggest (or not ?) creature of them all is a turtle which lives with the top of its shell above water surface. Those turtles have such a long lifespan that their shell is subject to erosion and a deep soil forms on top of it. You eventually get mountains, lakes, forests... A beautiful landscape and a nice background for a fantasy story.
If possible, I would also like the creature to move slowly (swimming or maybe walking on the ocean floor ?) such as once in a few centuries, civilizations from two different turtles can meet and exchange culture, genetic diversity and deadly diseases.
They don't need to be specifically turtles, or even looking like turtles if not possible. They however require to be alive and sentient, because I need them into my magic system (a mother nature sort of thing where you "ask" for magic).
Would such a creature be possible with as few handwavium as possible ? If absolutely necessary, magic is available, with whatever properties you need. The less magic the better.
The maximum turtle size I need is about 1000 - 2000 miles / 1500 - 3000 km. (that's Australia-sized) It's okay if only smaller turtles can move, and the biggest/oldest (dead-est ?) turtles are stuck to one place. If such a size is not possible, how large is the biggest possible turtle ?
How old would a turtle be in order to get nice erosion and landscape ?
What lanscape features would be available, or not, on such structure ? Small mountains, lakes... ?
For those two questions, keep in mind that we are on a waterball planet, with powerful wind and storms as explained in this question : Ocean and air currents on a water planet
If that's two many questions in the same post, or two broad, please let me know. And thank you for your time !