Depends if I'm using ice or not. If not, I'll fill it up a little higher to cool the smoke a bit more. If I am using ice, I'll fill it to just above the highest notch in my diffusor. More water = cooler hits. However, water, in addition to cooling the smoke, removes some of the THC in the process. You normally won't notice a difference, because you're taking bigger hits, and everyone knows you get the highest when you take the biggest hit you can and still hold it in for 7 seconds. More than 7 seconds, you need to take bigger hits. Less than 7 seconds, smaller hits.