How to get OnCollisionEnter working with Terrain. JavaScript

I am currently scripting a boat. I am using a box collider on the water, and a rigidbody on the boat so it looks like it's floating. There's also a Terrain collider on the terrain. I want my boat to stop when it hits the terrain, but right now I can't even get Unity to correctly detect that the boat has hit the terrain. Here's my code:

 var speed : float = 5.0;
 var rotateSpeed : float = 1.0;
 
 
 function Update () 
 {
 
    //Move the boat forward
    if(Input.GetKey(KeyCode.W))
    {
        transform.Translate(Vector3(1 * speed *Time.deltaTime, 0, 0));
    }
  
  
     // Rotate the boat
     transform.Rotate(0, Input.GetAxis ("Horizontal") * rotateSpeed, 0);
     
     
     //Plays Animation on the rod
     if(Input.GetKeyDown('space'))
     {
  var rod = transform.Find("Pole (4)");
  rod.animation.Play('Take 001');
     }    
 
 }
 
 
 function OnCollisionEnter(collision : Collision)
 {
    if(collision.gameObject.tag == "Terrain")
    {
       Debug.Log("Hit");
    }
 }