Since there is no out of the box volumetric navigation you will either need to implement it yourself in c++ or try fake the movement.
I assume you use blueprints only then faking it is the only real solution.
Here some thoughts how you could do it.
Have a pawn (or character) blueprint with a mesh component , set your shark as the mesh. The trick is that you should try and parent the mesh component to your scene root component and just modify its local transform on the Z axis to be at the height of i.e. its prey. So the shark could be “wandering” on the navmesh at height 10 by default but once it switches into attack behavior, you set its height (probably want to interpolate the value over time) to the height of its prey.
Hope that gives you some ideas to start with, good luck!