This business of fluorocarbon having the same refractive index as water is a load of tosh.
It has a refractive index much closer to water than nylon, hence it is "less visible", not invisible!
Generally speaking predatory fish have the best eyesight, they have to, to catch their prey.
Of the fish species in British freshwater, those with the best eyesight are:
Trout, Pike, Perch and Zander.
Chub also have very good eyesight. They can see you coming a mile away.
The fish with the poorest eyesight are catfish, tench, eels and bream.