Use spline Ik for the swimming motion and blendshapes for the mouth opening and fin movements. If you want to get a bit more creative you can make the tips of the sharks fins a softbody to get a sway action.
I've attached a small playblast of a fish test using spline IK so you can see what kind of movement a spline Ik can give you.
https://forum.simplymaya.com/showthre...?threadid=2676
I am enough of an artist to draw freely upon my imagination, knowledge is limited, imagination encircles the world. (Albert Einstein)
https://www.artstation.com/kurtb