Its called Fish because of a very old "tribal" merfolk deck that was popular in Urza Block Standard or Masques Block Standard. It featured a lot of merfolk and just locked down the opponent with blue control (freezing) and attacked with weenie merfolk (fish, or fish sticks). Thus, the deck was called Frozen Fish(sticks). Frozen Fish is how I remember it refered to as, though.
It was just what the creator named the deck, to have fun. However, it set a precedent that everyone followed like mindless automations, and it was: Anything featuring Merfolk has got to be called Fish. Sort of how like modern Sligh decks are nothing like Mr. Sligh's original deck.
Retarded, but that's the way it is. Because nobody is creative with deck names.