How to check if specific objects touch

So first let me point out I am a complete N00B so if I sound a bit dumb, I have an excuse.| So I am making an FPS and don't want to make it so that the character can go deep into water. So what I have if make a cube, put it above the character and made it a parent of the character. Then I added this script to the cube.

function OnCollisionEnter(Bam : Collision) { if(Bam.tag == "pond 2") { Application.LoadLevel(0); } }

This is supposed to make it so when the cube touches the water, the player goes back to the main menu screen. When I try it in the game it doesn't work though. The player just keeps walking. If anyone would help me that would be great!