you should add like knives and maybe you could even just use your fists to punch said fish.

NO WEAPONS

I believe the developers have taken a stance against many kinds of weapons, and I don’t think deadly weapons fit the game’s themes very well.
But setting that aside: I think it’d be very challenging to harm fish underwater with just your fist or something like a dive knife.
Being able to kill fish with just 1 slash of a knife and kill enormous organisms with one was pretty silly in my opinion.
Read the databank entry for survival multitool. There is an in-lore reason for why there are no weapons in the game.