I have observed many wild musk turtles basking. While it certainly is legit to argue they don't need UV, I would def provide it anyway. They will get the UV even if only from the water. It is very essential for vitamin d synthesis and I will argue it is better for the overall health of the turtle.
-Jesse , I would provide a UV source, and a :basking heat source. Musk turtles DO bask and most likely have the same needs as any other species.