This is a paid asset, but now you can download Underwater life deluxe for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
Underwater life deluxe v1.0
(Tested with Unity version 2020.3.23f1)
This package includes many different underwater animals (mostly fish) along with landscape rocks, plants and corals to decorate underwater scene. Animal assets are upgraded assets from previous Underwater packages released by myself – janpec , with improved textures and PBR support as well as improved animations. Most assets include AO, diffuse, and roughness textures for PBR calibration. Majority of animals have 5 animations (swim, fast swim, slow swim, eat and die) for basic range of underwater behaviour.