my guess:
First, some statements:
1. I believe a fish was once spotted in the water in a previous version of the game. Of course, this might be BS and just a joke/fake someone posted once.
2. There IS a shark model in the game
Based on the above, my complete and total guess is: TFP put the shark in as a "test model" OR as some odd way to keep people from from hiding in the water to keep zombies away from you... or maybe to make swimming 'dangerous'... much like using the gyrocopter (you're mostly safe) and having vultures (you can be attacked in the air if you're close to the ground). In the current game zeds can swim so you're not safe in the water (under most conditions).
So: I'm guessing TFP considered putting things in the water to attack you, and the "possible spotting of a fish" in the water may be part of this (or not) BUT from a technical perspective, and how the game has been progressing, I'm guessing there are no visible swimming fish because:
- You cannot normally see into the water unless you are in it
- The fish would just be burning up "animal counts" and causing resources to be used for something you'll likely never see
- TFP has not added "fishing" to the game yet, and it seems (based on this forum) they don't intend to
HOWEVER:
- There are some fishing mods, but all of them are "cast into the water and catch a fish". There are no actual fish in the water though:
https://7daystodiemods.com/?s=fish
- There is this "A Better Life" mod that adds fish and butterflies, etc but I don't think you can "fish" for them. and its for a18 and I don't believe works in a19?: