Term for "Death by Lack of Water"

Dehydration might be the word you're looking for:

He died from dehydration.

there's no term (at least to the best of my knowledge) for something like "He (word for died from thirst) to death"

Sure there is: "Died of thirst." The phrase gets more than half a million hits on Google, and over 100,000 in Google books.

There's also "died from thirst", but of seems to be the more common preposition.

If you really want a construction parallel to starved to death, though, you can simply say thirsted to death.

from dehydration. or if you want a longer form: of thirst.

To strictly fit the format, you could use thirst in its verb form. Hence

He thirsted to death.

However, its use as a verb is relatively rare (indeed, it may well be one of those interesting cases where metaphorical use out-numbers literal, and The athlete thirsted for Olympic gold seems more natural than I thirsted for a decent craft-brewed beer, though both events described are as likely to occur).

For that reason it's technically correct, but sounds unnatural, and I wouldn't recommend it. I'd go for keeping He died of thirst.

Hypernatraemia caused by dehydration as given in other answers, are the most likely direct causes of what actually dealt the body its final irrecoverable blow, but there's no verb form of hypernatraemia, and He dehydrated to death has the same problems as thirst as a verb, and is imprecise: diarrhoea is the second biggest cause of infant deaths, and a common cause of older deaths, and then it is also death by dehydration, but not of thirst. Indeed, a patient of dehydration may find it hard to drink as much as their carers are encouraging them to, as while they are dying from dehydration, they are not thirsty.

The proper medical term for dehydration is hypernatraemia; however there is not a special term for the terminal state of this condition... one would say 'death from hypernatraemia'

Hypernatremia is an electrolyte disturbance that is defined by an elevated sodium level in the blood. Hypernatremia is generally not caused by an excess of sodium, but rather by a relative deficit of free water in the body. For this reason, hypernatremia is often synonymous with the less precise term, dehydration.