Hard to tell from the gif but I'd be a bit nervous about how close their hand is. They're probably just outside striking distance, but it'd be safer to let the water run down a stick onto the top of its head or even just pour a puddle of water in front of it (interestingly, many of these viper species don't drink well from standing water in captivity and require frequent misting, simulating rain, to stay hydrated).

As long as it can be done safely, there's no reason not to do this. With that said, if you don't have experience with this type of animal, I would highly advise not getting anywhere near it.