"Marlin", the proper name and the fish, both find their roots in "Marlin", a shortened form of "marlinspike" which is a pointed iron tool used by sailors to separate strands of rope. Marlin is definitely NOT a variant of Marlon (Marlon finds its origins in the Old French word "merlin" meaning, "little hawk", or is possibly a variant of Merlin or Marcellus.)
https://forebears.io/x/forenames/marlin
As a Feminine name, It can possibly be a Spanish variant of Marilyn and Marlene.
https://www.behindthename.com/name/marilyn
https://www.behindthename.com/name/marlene
https://www.behindthename.com/name/marlen-2
https://www.behindthename.com/name/marlyn
This is an update to my previous comment via @Teddy01 on Sep/8/2020
Note to editors and admins: Please delete the comment originally by @Teddy01 on Sep/8/2020