OnTriggerEnter Not working

Hey guys this problem is aboute the function OnTriggerEnter it doesnt work and when i pud it in the Update it works only i need it to work when my player is in the water Collider What did i do wrong??? thanks

 var WaterLevel : float;
 var useGravity: boolean;
 var Player : GameObject;
 var useBlur : boolean;
 var Bubbels : GameObject;
 
 private var defaultFog = RenderSettings.fog;
 private var isUnderwater : boolean;
 private var normalColor : Color;
 private var underwaterColor : Color;
 private var defaultSkybox = RenderSettings.skybox;
 var noSkybox : Material;
 
 function Start () {
     normalColor = new Color (0.5f, 0.5f, 0.5f, 0.5f);
     //0.22f, 0.80f, 0.80f, 0.5f)
     underwaterColor = new Color (0.80f, 0.85f, 0.75f, 0.5f);
     RenderSettings.fog = true;
     Bubbels.active = false;
     camera.GetComponent(Blur).enabled = false;
     camera.GetComponent(Fisheye).enabled = false;
 }
 
 function OnTriggerEnter (Player : Collider){
     if ((transform.position .y < WaterLevel) != isUnderwater) 
     {
          isUnderwater = transform.position.y < WaterLevel;
          if (isUnderwater) SetUnderwater ();
          if (!isUnderwater) SetNormal ();
     }
 } 
 //function Update () {
 //   if ((transform.position .y < WaterLevel) != isUnderwater) 
 //    {
 //         isUnderwater = transform.position.y < WaterLevel;
 //         if (isUnderwater) SetUnderwater ();
 //         if (!isUnderwater) SetNormal ();
 //    }
 //}
 
 function SetUnderwater() 
 {
    //The Bubbels are a particle renderer Attached to The Player
    Bubbels.active = true;
    RenderSettings.fog = true;
    RenderSettings.fogColor = underwaterColor;
    RenderSettings.fogDensity = 0.10f;
    RenderSettings.skybox = noSkybox;
    camera.GetComponent(Blur).enabled = true;
    camera.GetComponent(Fisheye).enabled = true;
 
  }
 
 function SetNormal() 
 {
    Bubbels.active = false;
    RenderSettings.fog = defaultFog;
    RenderSettings.fogColor = normalColor;
    RenderSettings.fogDensity = 0.03f;
    camera.GetComponent(Blur).enabled = false;
    camera.GetComponent(Fisheye).enabled = false;
 }
     // Turn this collider into a trigger on startup
     collider.isTrigger = true;