Live fish can be observed swimming in large bodies of water, as expected. They cannot be interacted with and are therefore considered scenery as well as a creature, similar to the birds from Fallout 3.
Fish do not appear in any Fallout game other than Fallout: New Vegas and Fallout 4, except for the cut fish that were meant to be a consumable item in Fallout 2.
Fish were mentioned by an unnamed fisherman in random coastal encounter in Fallout and by San Francisco citizens in Fallout 2 as food.
Fish in Fallout: New Vegas appear to be virtually unaffected by radiation, possibly due to the low level of background radiation in the Mojave area. However, according to various wastelanders' account in the first two Fallout games, many fish caught in Pacific Ocean were mutated by radiation, showing abnormalities such as three eyes, or gigantic physical size.
When the Courier asks Cass why she took up caravanning, she responds by saying she "took to it like a fish in water." The Courier has the option to say "What the hell is a fish?" She replies that a fish is like a slimy lakelurk thing that doesn't have claws and doesn't attack people. She also describes it as a bird that stays underwater, and that some of them have legs. Other dialogue options lead to Cass saying that she has never seen a real fish, only a plastic one on a wall in a bar which used to sing (referencing the Big Mouth Billy Bass), according to the bar owner and much to Cass' disbelief.