Answer:
Typically, divers use "booties" that are made of neoprene. That is the same material wetsuits are made of. They usually have rubber on the bottom of them so you don't slip around, but they are made to wear while wearing fins. You put the booties inside the fin for maximum comfort while diving. They're usually about 20-30 bucks at the dive shops and last about 5 years assuming you dive about 20 times a year.