Ok Ive fixed it with the help of WesleyE. What I did is I set up a box collider underneath the water plane and named it underwater and then applied my code to it which is:

 //set your varibles here.  var fog = false;  var fogColor = Color (0, 0.4, 0.7, 0.6);  var fogDensity = 0.04;  var skybox : Material;  //this is where you go underneath the water or in this case into the trigger aera  //this is just the effect not anything to do with force.  function OnTriggerEnter(other : Collider) {  fog = true;  RenderSettings.fog = fog;  RenderSettings.fogColor = fogColor;  RenderSettings.fogDensity = fogDensity;  RenderSettings.skybox = skybox;  }  //this is where we are exiting the water and the effect of being underneath the water is ended.  function OnTriggerExit(){  fog = false;  RenderSettings.fog = fog;  }

Hey never mind it was so simple all I had to do is put it in there lol and I found a previous question that has a script that allows me to make the underwater effect thanks anyway :D

The script for underwater I am using now is

 var fog = false;
    var fogColor = Color (0, 0.4, 0.7, 0.6);
    var fogDensity = 0.04;
    var skybox : Material;

    function OnTriggerEnter(other : Collider) {
        fog = true;
        RenderSettings.fog = fog;
        RenderSettings.fogColor = fogColor;
        RenderSettings.fogDensity = fogDensity;
        RenderSettings.skybox = skybox;
    }

the problem now is that when I leave the water it keeps the effect on Any Ideas On how to fix this