Ability to control mob's swimming behavior to that of a water mob/more mob AI bases

So I want to make a fish mob, right?

And even after i set everything to "Water Mob", "no drowning", etc. there is no way to control swimming function.

Right now it's either the mob has no ability to swim and it sinks to the bottom, or, when you add "Mob swims in water" as an AI behavior, it floats at the top.

So it would be great if there was a way to fix this, and allow the mob to swim under the water like a fish mob or dolphin. 

If this is too hard, maybe could the Fish mobs' AI be added as an AI base?