If you are the DM, you will really need a good reason to force everyone to pay that close attention to water during a play session and you should make sure players are aware that water rationing will play an important part of the session/campaign. Otherwise, the issue will become a source of frustration.

That said,

Characters need to be able to readily consume water to stay hydrated. "Access" means that the requisite amount of water exists and that the character is able to drink it in some way (eg, not chained up at a distance).

First, if a character has less than the necessary amount of water, they should be alerted to the fact that they need to find a source of water at some point during the day.

Failing this, if it's less than 100 degrees, at the conclusion of a day a character is either

• Forced to roll for exhaustion if they had at least half a gallon at the start of the day
• Suffers a level automatically if the initial amount was less than half a gallon

If the temperature is greater than 100 degrees (extreme heat), the characters should be directed to take stock of their water at the start of the day and given a timeframe in which they need to locate a source of water.

If a source is not found in that time, the result can be handled in one of two ways, depending on how important the timing of the exhaustion checks is.

• If the DM intends to have the exhaustion factor into the day's events as a means of increasing the difficulty and adding to the dramatic tension, the rolls take place every hour after the established amount of time has elapsed.
• If the exhaustion is not intended to be used in this way, the rolls might be done in series once the day ends or a water source is finally located, depending on how many hours elapsed after the water ran out. The exhaustion levels are then carried over the next day(s) until enough rest has accrued for it to wear off.

Yes, you can resurrect him.

The resurrection spell says (emphasis mine):

You touch a dead creature that has been dead for no more than a century, that didn't die of old age, and that isn't undead. If the soul is free and willing the target returns to life with all its Hit Points.

Under normal circumstance I would argue that a destroyed/killed undead would no longer count as undead but Jeremy Crawford has posted a call on the similar revivify spell:

If you cast revivify on a zombie, the creature returns as a zombie. The spell doesn't change creature type.
- Jeremy Crawford

This call would let me extrapolate that it would not be possible. Except that this specific case is actually covered by the MM:

Once turned into a zombie, a creature can't be restored to life except by powerful magic, such as a resurrection spell. - MM pg. 315

This would mean the specific case of a resurrection spell (or if you want to be absolutely sure a true resurrection spell) it is able to turn a destroyed zombie back into the person it was before originally killed.

Though you might want to destroy the zombie outside of the temple and carry him the last part as I am guessing that most temples are not happy with zombies walking in.

Beyond RAW

This case relies on a lot of specific rules and exceptions if tis would come up at my home game I would allow resurrection of mindless destroyed undead like skeletons and zombies. I would not allow revivify even if the spell was cast before a minute has past since the original death.