I'm not aware of a spell for rehydrating someone who became dehydrated on Tisha B'Av. So we must all be vigilant not to requre such a spell and to stay cool and hydrated and aware of the signs of dehydration. Here is a primer:

The Danger of Dehydration on Tisha B’Av

Dehydration can be life-threatening, and must be regarded with the utmost of seriousness by any Jew who fasts on Tisha B’Av. By recognizing the symptoms of the various stages of dehydration, and acting responsibly, we hope to all complete the fast in a safe and meaningful way.

The first thing to remember is not to engage in activities that will increase the likelihood of dehydration. For many people, the wisest course of action is to stay indoors, in an air-conditioned environment, and avoid strenuous activities that can contribute to dehydration. Finishing the fast is more important than going to shul to hear Eicha or Kinos, which can be said at home.

Nursing mothers are especially advised to stay indoors, and their husbands are urged to help with childcare so that their wives can minimize their exertion.

Below are the symptoms associated with the various stages of dehydration, in ascending order of seriousness: (My thanks to Dr. Robert Silverman for his help in preparing these guidelines.)

1. Mild

Increased thirst, decreased urination, dry mouth

2. Moderate

Headache, Irritability, Lightheadedness when sitting or standing (goes away when lying down), extreme thirst, dry appearance inside the mouth and the eyes don’t tear, decreased urination—urine is dark amber or brown.

THOSE WHO ARE FRAIL OR HAVE SIGNIFICANT MEDICAL PROBLEMS SHOULD BREAK THEIR FAST AND SEEK TREATMENT

3. Nausea, weak rapid pulse

THOSE WHO ARE ELDERLY OR HAVE CHRONIC MEDICAL PROBLEMS SHOULD BREAK THEIR FAST AND SEEK TREATMENT

4. Severe – Life-threatening

Altered behavior (confusion, severe fatigue), no urination, feeling faint even lying down, fainting, cold clammy skin, or hot dry skin.

IMMEDIATELY BREAK YOUR FAST AND SEEK TREATMENT!