All turtles require sunlight. Enough said. Its better to have a basking spot and not find out if the turtle will get shell rot. Giving the animal the option and observing is the best bet. Cork takes up absolutely no swimming space but the turtle may get too heavy for it. You could also try a turtle dock.
First off, the florida softshell (Apalone ferox) and the fly river turtle (Carettochelys insculpta) are two completely different species, different genera, different parts of the world...what works for one does not work for the other.
As far as needing the normal UVA/UVB light that most basking turtles need, is questionable for FRTs. Penetration of UVB light through water drops off severely after the first few inches, so for a turtle that never leaves the water, it is questionable that the desired spectrum is even reaching them enough to have an effect.
Also, since FRTs do not bask, the inclusion of a basking area would be pretty wasteful; you would be better off with the extra water volume. Most keepers recommend some sort of structure near the surface though as a resting area.