To get started, simply drag and drop a fish prefab into your scene. Upon playing, the fish will swim around the scene, avoiding any colliders.
You can find the fish prefabs in:
Cosmocat\InstantFish\Prefabs
Important
If you modify a prefab be sure to save it as a new prefab (or prefab variant) outside of the Cosmocat directory.
By default, the water level is 0 on the Y-axis. The fish will not swim above this level. If you would like to override this, add a WaterLevel component to your scene and adjust as needed.