The human body contains approximately 60% water on average. (According to the USGS). The spell Create or Destroy Water states: "You destroy up to 10 gallons of water in an open container within range." In addition, "When you cast this spell using a spell slot of 2nd level or higher, you create or destroy 10 additional gallons of water". A gallon of water weighs roughly 8.35 pounds.
Say there's a human who weighs 200 pounds. 60% of 200 is 120 – approximately 120 pounds of water are in this person. dividing 120 by 8.35 = around 14 – so those 120 pounds of water are, in other words, around 14 gallons. The destroy water spell can destroy up to 20 gallons.
According to these measurements, "destroy water" could theoretically completely dehydrate a 200 lb human.
The biggest argument against this in my eyes is that the water must be in an "open container". However, the human body is technically the container of the water. Thus, if a barbarian takes a big slash across the torso, resulting in a large open wound, the wounded human may, technically, count as an "open container".
Now, the 64 million dollar question – would "destroy water" dehydrate a human with an open wound, removing all water from their body and instantly killing them?
Best Answer
Allowing you to target a person makes the phrase meaningless
What is a "container"? Merriam-Webster says,
Is a human a receptacle for holding water? The wording of the definition suggests that there is some intent--that the object is intended to hold water, in this case. Do people keep humans around to hold water? That seems unlikely.
However, let's say that humans are valid "open containers". Then, what is not an open container? Sure, the human's blood is "open," in some sense, but the vast majority of it is still inside the body. If we say that the wound makes that "open," then basically any gap is "open". Therefore, literally everything that has any water in it is an "open container". Heck, why use the level 4 spell Blight to kill plants, when plants are "open containers"?
Finally, consider game balance. Should a 1st level spell be an instant kill on any humanoid that's taken damage? Probably not. You can often do crazy things with magic in D&D, but it is ultimately a game, and not a physics simulator.