Minn of the Mississippi
I never read the book, just looked at the pictures. Seems a large terapin, not to be confused with a narwhal, lived in a secluded maze of shrubs underwater in a printshop boat that sank like a Spanish gallion full of gold, bronze, and tin.
First, moss grew on the turtle and he swam through warm summer currents to thin out the moss. A few chipper minnows nipped and clipped at the grass until it wasn't much of an earth-map, clean & green.
The shell will go soft if undernourished. Found one under the dresser once, eating playdough. Took him up to Angola for the cure.
So here's this big tortoise, you see, holding court over at the Mainside Dock Bar, in full splendid Neptune regalia - I tell you the muther was brisk! Had a gold watch and he's knock a lobster upside the head with it. What do sea creatures need to tell time for? They don't.
I think you could ride on Minn the Turtle, and sail around the Cape. I forget if he had children, or what. What the whole point was, I can't remember. He was a sleek-shelled horned-eared old Captin of a turtle, though.
Now see the real Minn