The best replacement for fins is fcs.

The best alternatives to fins are rated as following:
★★★★★  fcs
★★★★  flippers
★★★  scales
★★★  regular fins
★★  no fins
★★  wings
★★  fins
★★  power pro
★★  long fins
★★  dolphin fins
★★  phins
★  rocket fins
★  shark fins
★  feathers
★  open heel
★  scuba fins
★  aluminum fins
  paddle fins
  short fins
  rubber fins
  jet fins
  thrust vectors
  full foot fins
  split fins
  thruster
  future fins
  normal fins

Your alternative for fins is not listed? Suggest!
Speaking about fins, one probably shouldn't forget about surfboard.

Other search terms related to fins and their ratings are:
★★★★★ surfboard
You have in mind a companion for fins and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of fins in all years. Click on graph for more.
fins
fcs
flippers