You could have a collider set as trigger that upon collision initiates a sequence on the player.

//Trigger collider script
function OnTriggerEnter (other : Collider) {
    if (!other.CompareTag("Player") return; //Exit if this isn't the player
    var pScript : PlayerScript = other.GetComponent(PlayerScript); //Get the script component of the player (this could be cached for performance too)
    pScript.Kill(); //Run the function called Kill on the player
}

//Part of the player script
private var startPos : Vector3;
function Start () {
    startPos = transform.position;
}

function Kill () {
    animation.Play("death"); //Initiate the 'death' animation
    yield WaitForSeconds (animation["death"].length); //Wait for animation to finish
    transform.position = startPos; //Reposition the player at starting point
}

If you don't use the physics engine you might as well want to check for a certain height, and when below that you initiate the kill function.

private var startPos : Vector3;
private var waterLevel : float = .0;
private var isDead : boolean = false;

function Start () {
    startPos = transform.position;
}

function Update () {
    if (transform.position.y < waterLevel && !isDead) Kill();
}

function Kill () {
    isDead = true;
    animation.Play("death");
    yield WaitForSeconds (animation["death"].length);
    transform.position = startPos;
    isDead = false;
}