C4 makes fins for serious freedivers, spearfishermen, and mermaid cosplayers. And also people who just want to swim really, really fast. The carbon fiber designs tout themselves as key players in more underwater-themed championships and world records than any other fin on the market. C4, an Italian company, is also the largest producer of carbon fiber fins in the world.
Their fins are made of T700 carbon fiber, said to be 40% stronger and more resistant to wear than the T300 strand used in standard carbon fiber fins. C4's American distributor, Technosport, carries about a half dozen models/generations of the fins.
I emailed C4 to ask if wearing carbon fiber fins would make me faster than Michael Phelps, and they said no, so I said what about a whole carbon fiber suit, and they said probably not, so I said what about if he's high? Still awaiting a response.