Aquatic mobs do not appear to be spawning in superflat worlds, unless they appear in a structure such as ocean monuments. This is noticeable when using the "Water World" preset, or any superflat world with an ocean/river biome and water on the surface.
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Superflat waterworld with additional stone (64) and less water (30) also lack ocean mobs (fish), only drowned. Subterranean ocean monuments are created with structures on, dry, but with elder guardians.
update to 25w02a set Hard to effectively Peaceful...somehow. Game setting/file is still Hard, but behavior is Peaceful.
gaspoweredpick
Can confirm:
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