Fish swim in oceans in Spore.
Cell Stage Edit
In the last level of the Cell Stage, the Cells are probably fish. The fish as seen later in the game seem to be made in the Early Creature Creator, with three eyes, primitive mandibles, and cilia for fins. This may be considered evidence that large cells such as Snorf and Bloato are actually small fish.
Creature Stage Edit
Tribal Stage Edit
In the Tribal Stage, fish can be harvested for food. Fishing is the second easiest method for collecting food, because the tribe usually has to go long distances to reach a suitable fishing spot. With the Flying Fish ability, your chieftain can summon fish; the amount of fish summoned depends on the tribe's size. If your creatures are herbivores, they will automatically harvest seaweed instead.
Other Stages Edit
Fish don't appear in the Civilization or Space Stages except for when tribes are seen fishing (only in the Civilization Stage) or when fish are on tribes' food piles (in both stages). They don't appear in the Sporepedia, either.
Galactic Adventures Edit
In Spore Galactic Adventures fish can be placed into adventures as an effect, but they cannot be interacted with. Due to the nature of effects, the fish can be placed hopping out of rock, or lava; this can be used for humorous adventures. They are identical to the fish in the Tribal Stage: they look the same, and all they do is hop in and out of the water.