That's probably not the best way to top off the tank.
People do some sketchy things with rental cars, and some even like to brag about it. However, we’ve never seen someone put water in the fuel tank on purpose, in an attempt to raise the gauge before returning it to the rental company. That is, until now.
We don’t have much information on this alleged activity, save for what we see in the video. The car in question is a Nissan March, and if we had to guess a location for this caper we’d bet on Brazil based on the license plate. It appears the lady is using a watering can, though she’s obviously not tending to flowers. When she steps away the gas cap is easily seen dangling from the open fuel door, so she was clearly putting something into the tank.
The video description says the woman had “already spent the gasoline” which we’ll interpret as meaning she already gassed up the Nissan, but apparently not enough to bring the gauge up to the full mark. As per standard rental car procedures, you return the car with the same amount of fuel it had when you picked it up, which is usually a full tank. We’ve all played the unspoken game of topping it off just enough so the gauge hits the full slash, but presumably this person either misjudged the mark or ended up driving a few extra miles after the fuel stop.
If the video title and description is in fact true, an epic facepalm is our only response. Perhaps she thought water will simply dilute the fuel, but the two don’t mix in the least. Even a little condensation in the tank can cause problems, but any significant amount – such as pouring enough into the tank to move the gauge – can pretty much shut the car down. The only fix at that point is to flush the entire system, and that typically involves removing all kinds of bits, as explained here.
The video abruptly cuts off just as the woman spots the camera, so we’re guessing some kind of exchange took place at that point. Per the video description the rental company is aware of the woman’s actions, but at the very least she’ll be facing a public shaming.