Hi, believe it or not - I have some experiences with the func_fishpool! LOL.
It's been a while but let's see what I can remember, but I'm currently in a coffee shop on a mac so I can't open the entity up to be more specific. maybe later.
1. When you set it up - is radius based - meaning everything happens around in - you are in the center of a circle possibly sphere.
2. There are no fish models that I can recall - I may be wrong about this as they may be embded into the TF2 Source code so you may need to download the fish models and reference them in.
3. AI - yes they swim sort of - the models will as far as i can remember - pivot, wiggle, on a central point to give the impression of swimming.
4 AND THE MOST IMPORTANT - THE FUNC_FISHPOOL IS AN ENTITY LIMIT KILLER!!!! To explain - yes it is one entity - however all the fish that it creates are separate entities! If you have lots of room in terms of entities go ahead and use it, if no then don't this one item can easily put you over the 1600 limit.
However, it still is fun to use! and you might be able to use it for other things - ie. asteroid field