Answer

In Jain practice, during Āyambil / Ekāsana-type tapas, water should generally be taken only during the permitted daytime period, after sunrise and before sunset, and according to the rules of the specific vow you have taken.

If you are doing Ekāsana, then traditionally:

• You eat only once in one sitting.
• Water is usually taken according to the rule you have accepted for that tapas.
• Many people drink water during that same sitting, while some traditions allow boiled water during the permitted daytime period.

Important Clarification

The exact rule can differ by Jain tradition, family practice, and the guidance of your Guru or temple tradition.

So the safest answer is:

Drink water only in the manner and time allowed by the specific Ekāsana vow you have taken, and avoid water after sunset.

Spiritual Understanding

The purpose of Ekāsana is not only limiting food, but also developing:

• self-control,
• awareness,
• reduction of attachment to taste,
• discipline in eating and drinking.

Takeaway

For Ekāsana, drink water only during the permitted time, preferably as guided by your Guru or local Jain tradition, and do not drink after sunset.