Smooth Y rotation based on X rotation (C#)

Hello, I am trying to simulate a shark swim inside water.

At the moment I can get it to move and rotate based on its velocity.

But my rotations on Y aren't smooth because they change on condition and so they aren't relative.

My game is 2.5D so the character (a shark) won't move in the Z axis.

If you want to understand better the rotation I want to achieve you can check this video of Hungry Shark that moves exactly as I would like to:

Hungry Shark Video

Here is my code for the moment:

 public class UnitPlayerMove : MonoBehaviour
 {
     
     public float speed;
     private bool isFlying;
     public float flyingForce;
     private float baseY;
     private float angleZ;
     
     // Use this for initialization
     void Start()
     {
         angleZ = transform.rotation.eulerAngles.z;
     }
     
     void Update()
     {
         float moveHorizontal = CFInput.GetAxis("Horizontal");
         float moveVertical = CFInput.GetAxis("Vertical");
         
         float CFAngleX = CFInput.ctrl.GetStick(0).GetAngle();
         float angleX;
         float angleY;
         
         if (CFAngleX >= 0f )
         {
             angleY = 270f;
             angleX = -(CFAngleX - 90f);
         } 
         else
         {
             angleY = -270;
             angleX = -(-CFAngleX - 90f);
         }
 
         
         Quaternion targetRotation = Quaternion.Euler(angleX, angleY, angleZ);
         
         transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, Time.deltaTime * speed);
      
         
         Vector3 movement = new Vector3(moveHorizontal, moveVertical, 0.0f);
         
         rigidbody.velocity = movement * speed;
         
         rigidbody.position = new Vector3 (
             rigidbody.position.x,
             rigidbody.position.y, 
             1.0f
             );
         
     }
 }

PS: CFAngleX is the angle obtained by a touch stick plugin, the joystick was oriented in a strange way and so I've made this little part of code you saw to get a normal X angle.

If you want an idea about how to achieve a smooth Y rotation based on the X one (or something else if I am on the wrong way), please let me know.

In advance, thank you.