Water Flow is a bottled water brand that appears in GTA 6.
The brand was introduced in GTA 5, where advertisements for the water appeared on both the radio and as a building wrap on a bank in Downtown Vinewood.
It was confirmed to return in GTA 6 via Trailer 2, which revealed a 
Since its last appearance in GTA Online, the Flow Water bottle has received a brand-new label design.
The water comes in multiple flavors, including Ginger Tang, Tea Twist, Lava Chili, and Zero Lo-Cal. Given the name of the final Water Flow flavor, it's likely that the other options have more calories than standard flavored water.
The company also appeared as a sponsor of select racing vehicle liveries in GTA Online.
Water Flow appears to be a reference to the real Vitaminwater, which features a similar array of flavored water products.
