Trouble Setting/Combining LayerMask with "Builtin" Layer

Hello everyone, I'm having an issue trying to set a multi-layer LayerMask in code, but have been getting some results I don't understand. I have read This post, so I thought I had at least some understanding of how this should work, but clearly I'm not quite there yet.

In Detal:

The issue comes into play when I try to set/add a builtin Layer to the mask. In the code below, the 3 user set layers show up correctly, but the builtin "Water" layer is nowhere to be seen:

     public int water = 4;
     public int npcComp = 8;
     public int playerComp = 9;
     public int terrain = 10;
     
     public LayerMask hittableLayers; //public just for easy display.
 
 
     void Start()
     {
         hittableLayers = (1 << water) | (1 << npcComp) | (1 << playerComp) | (1 << terrain); //returns as 'npcComp + playerComp + terrain'.
     }

If I use the NameToLayer() method, everything seems to work:

     void Start()
     {
         hittableLayers = (1 << LayerMask.NameToLayer("Water") ); //returns as 'Water'.
     }

If I use "4" directly, everything seems to work:

     void Start()
     {
         hittableLayers = (1 << 4); //returns as 'Water'.
     }

But, if I set a int Var to 4 and use that, I get back "Unnamed6" as the layer:

     void Start()
     {
         hittableLayers = (1 << water); //returns as 'Unnamed6'.
     }

So I'm wondering what the difference is in those cases. Why does using "4" work, but using a int with the value of 4 doesn't (because it seems to work for user layers)? Any ideas are appreciated, thank you for reading.

UPDATE:

It seems to matter if "water" is a private (it works) or public (gets 'Unnamed6') variable, not sure why...