^^ that is not water in the sight glass - what you see moving is liquid refrigerant and when low you will see bubbles - if you see foam you are extremely low.
A normal system looks like this
bubbles on A/C compressor startup - and go away as the temperature settles in the evaporator
normaly clear when A/C compressor is running and vents are blowing cold
when compressor cycles off and ON again - you will see bubbles for a second or so (thats the expansion valve working)
A completely empty system should not operate the compressor because of the low pressure safety switch - unless your A/C waala idiot has hotwired the A/C without a low pressure switch