Can't sell water? Ok, here's some free water with your peanut purchase

You are on the right track but it is more malicious than that.

The drug deals done in dance clubs is generally the clubs own product being sold. In every jurisdiction in the US essentially, establishments that serve alcohol are by law required to serve free water. If some suckers spend the $6 on water, cool, but otherwise, those water bottles are the best way to hide drug profits.

Ever wonder why DJs are spraying the crowd down with bottled water from the bar and it isn't just part of the clubs set design? They gotta get rid of the inventory.