In Jain Ekāśan (one-sitting/one-meal in a day), follow this simple, safe rule for water:

• Start only after sunrise. Do not drink before sunrise or after sunset.
• Drink only boiled and properly filtered water.
• Best (śuddh) way: take water only with your one meal, in that single sitting.
• If your maryādā allows between-meal water, take only boiled/filtered water during daylight, with due pachchakkhān (vow) and with proper gaps; never at night.

Tradition notes

• Śvetāmbara: Common maryādā permits only boiled, filtered water in daylight; the strict form of Ekāśan takes water only during the meal.
• Digambara: In Ekabhukti (one meal), ideal is also to limit water to the meal; some follow daylight-only, filtered-water allowance as per their ācārya’s maryādā.

If you are observing under a specific gacch/saṅgh/ācārya, follow that maryādā exactly.