Depending on your needs, you may find the topic below helpful:
The solution to that topic is, however, not concerned with the usage of the gravity overrides of Area3D. Instead, it computes the velocity that achieves constant circular motion i.e. gravitational acceleration is not considered. Given that you’re looking to implement a fish “swimming at a single depth”, I reckon the topic will be useful to you. However, in order to modify the “swim-depth” at run-time, the solution presented in the topic must be modified.
Let me know if you got any questions.