Skybox not changing (through code)

I tried to change the skybox through the code, and only the "normal" skybox is applied in game. The other one isn't... Here is the code :

 using UnityEngine;
 using System.Collections;
 
 [System.Serializable]
 public class Underwater : MonoBehaviour {
     private float waterLevel;
     private GameObject water;
     private bool isUnderwater;
     public Color normalColor;
     public Color underwaterColor;
     public Material normalSky;
     public Material underwaterSky;
 
     // Use this for initialization
     void Start () {
         water = GameObject.Find ("Daylight Water");
         waterLevel = water.transform.position.y;
 
         normalColor = new Color (0.5f, 0.5f, 0.5f, 0.5f);
         underwaterColor = new Color (0.22f, 0.50f, 0.77f, 0.5f);
 
         RenderSettings.fog = true;
         SetNormal ();
     }
     
     // Update is called once per frame
     void Update () {
         if ((transform.position.y < waterLevel) != isUnderwater) {
             isUnderwater = transform.position.y < waterLevel;
             if(isUnderwater) SetUnderwater ();
             else SetNormal ();
         }
     }
 
     void SetUnderwater () {
         RenderSettings.fogColor = underwaterColor;
         RenderSettings.fogDensity = 0.03f;
         water.transform.rotation = Quaternion.Euler(180f, 0, 0);
 
         RenderSettings.skybox = underwaterSky;
     }
     
     void SetNormal () {
         RenderSettings.fogColor = normalColor;
         RenderSettings.fogDensity = 0.002f;
         water.transform.rotation = Quaternion.Euler(0, 0, 0);
 
         RenderSettings.skybox = normalSky;
     }
 }

I'm a newbie in Unity and with this website, btw. Thanks a lot.