A word that means "to deprive someone of water"

Solution 1:

Merriam-Webster does have a closer definition than you have mentioned for "Dehydrate:

2: to deprive of vitality or savor

"Vitality" here could very easily refer to water, depending on context.

Also, there is a medical definition of "Desiccate" which means according to MW:

to dry up or cause to dry up : deprive or exhaust of moisture;

or the less active, non-medical definition:

1: to dry up

Solution 2:

Seems like you can actually be "starved" of things other than food...

1. To suffer or die from extreme or prolonged lack of food.
2. Informal To be hungry.
3. To suffer from deprivation: a puppy starving for attention.

"Starved for attention" as in the third definition above, or "starved of oxygen" for example.

So, as in this article, they use "starved of water" to mean being deprived of water:

One in eight dying patients are starved of food and water in their final days of NHS care.

And this book uses "water-starved" as part of its title... surely a book about being starved of water would have found an official word if there were one:

Let There Be Water: Israel's Solution for a Water-Starved World

So, for the sake of your request, it seems like "water-starved" is a good choice.