Check mouse click in front of object

I have a boat, the boat has cannons on both sides. Cannons are prefabs. I need a script that checks if the mouse click (fire cannons) is in front of the cannon or not, so that it only fires on the side where the mouse is in front of the cannons. The game is top-down, if that matters.

This is what i got so far but it doesn't collect the position data.

 var partSys : ParticleSystem;
 var mpox = 0.0;
 var gpox = 0.0;
 
 
 
 function Start () {
     
 //temporary particle system to test fire
 partSys = gameObject.GetComponent(ParticleSystem);
 
 }
 
 function Update () {
 
 //for debug purposes
 mpox = Input.mousePosition.x;
 gpox = transform.position.x;
 // Tried like this but still nothing
 // mpox = Camera.main.ScreenToWorldPoint(Input.mousePosition).x;
 // gpox = Camera.main.ScreenToWorldPoint(transform.position).x;
 
 
 
 //this is where i'd do the checking, but none of the "position.x" gets anything
 //also no errors, just plain zeros
 //it fires the cannons on both sides
 if(Input.GetMouseButtonDown(0) && Mathf.Sign(gpox * mpox) == Mathf.Sign(gpox)){
     partSys.Emit(1);
 }
 
 }