Classic Fireball Shooting Up Obstacle

I am trying to make a "fireball" (in this case water) spawn every 3 seconds, shoot straight up, and fall back down. Like the classic fireball obstacle in old Mario games.

With the code I have so far, it just sends the waterball straight up and it never comes back down. Also, another one does not spawn after 3 seconds. Coding is not my thing so if anyone could help that would be great.

 using UnityEngine;
 using System.Collections;
 
 public class waterBall : MonoBehaviour {
 
     public float shotDelay = 3f;
     public Projectile projectile;
 
     private float shotHeight = 20;
 
     // Use this for initialization
     void Start () {
         if (shotDelay > 0) {
             StartCoroutine(OnFire());
         }
     }
     
     // Update is called once per frame
     void Update () {
         if (projectile) {
             Projectile clone = Instantiate(projectile,transform.position, Quaternion.identity) as Projectile;
         }
         GetComponent<Rigidbody2D>().AddForce(new Vector2(0, shotHeight));
     }
 
     IEnumerator OnFire(){
         yield return new WaitForSeconds(shotDelay);
         StartCoroutine(OnFire());
     }
 
 }