Pour in at least enough water so that it covers the bottom of the metal shaft.
Cool the smoke idea:
If the smoke gets harsh, sometimes its good to lightly blow out some accumilated smoke from the water bowl.
Add ice cubes but refrain from freezing in refrigerator as it could easily crack the bowl.
Cracked or broken water bowl : I've used rubber cement on a glass vase that broke in two complete parts after I tried freezing water in it. Ruber cement left for a few hours made a perfect vase again that I use often with zero drippage.
You can even fill a very large container or can with ice or dry ice surrounding the hookah for long lasting cool smoke - keep in mind that if too much water around the hookah accumilates, the hookah will start to float and fall over from the air inside the glass bowl, water displacement.