drunk by a human or drunk at all? I'd suspect that not all water has been drunk at all, but lets cover the human part.

Lets say the average human drinks 3 liters per day and lives 80 years (3 * 365 * 80 ) = 87,600 liters per human.

there have been about 100 billion humans so 100 b * 87600 =8,760,000,000,000,000 liters have been drunk.

there are about this many liters in Earths 5 oceans (source google search): 1,400,000,000,000,000,000,000

Assuming no one has ever drank the same amount of water before, 8,760,000,000,000,000 < 1,400,000,000,000,000,000,000

It sounds fairly certain that not all water has been drunk by a human, even if my math is way off