Short answer: During Paryushan, the ideal and traditional practice is chauvihar—no food or water after sunset until the next sunrise. So at night, you should stop drinking water from the moment of local sunset.

Details:

• Shwetambar practice in Paryushan: Chauvihar is observed; water is to be stopped exactly at sunset and resumed only after sunrise.
• Digambar practice (during Das Lakshan which follows): The same principle is followed—no water after sunset.

Notes and allowances:

• If someone is ill, on medicine, very elderly, or a small child, minimal boiled-and-filtered water may be taken strictly as aushadh (medicine), with inner repentance (pratyakhyan) for the exception.
• On ordinary days some laypeople follow tivihar, but during Paryushan most adopt chauvihar.
• Practical tip: Check your local sunset time for the day and finish your last sip a few minutes before sunset.

This preserves ahimsa (avoiding harm to night-borne organisms) and the purity of vrata during Paryushan.