How can I rotate my 'shark' vertical?

I am trying to control a shark. I am able to get it moving just how I want and rotating more or less how I want. But, after processing user input, I want the shark to be vertical. But, I can not figure out how to set the z rotation to 0.0f after the input has been processed.

 using UnityEngine;
 
 public class Shark : MonoBehaviour
 {
     [SerializeField]
     private float rotationSpeed;    
 
     private float inputHorizontal;
     private float inputVertical;
 
     private void Update()
     {
         inputHorizontal = Input.GetAxisRaw("Horizontal");
         inputVertical = Input.GetAxisRaw("Vertical");
 
         ProcessInput(inputHorizontal, inputVertical);
 
         RollVertical();
     }
 
     private void ProcessInput( float horizontal, float vertical)
     {
         Quaternion currentRotation = this.transform.rotation;
         float horizontalRotation = horizontal * rotationSpeed * Time.deltaTime;
         float verticalRotation = vertical * rotationSpeed * Time.deltaTime;
         Vector3 rotation = new Vector3(verticalRotation, horizontalRotation, 0);
         Quaternion newRotation = currentRotation * Quaternion.Euler(rotation);
 
         this.transform.rotation = newRotation;
     }
 
     private void RollVertical()
     {
         Quaternion roll = Quaternion.Euler(transform.rotation.x, transform.rotation.y, 0.0f);
         transform.rotation = roll;
     }
 }