Short answer:

• If you took Chauvihar pachkhan: you must wait till Navkarsi to drink water.
• If you took Tivihar pachkhan: you do not have to wait till Navkarsi for water; boiled water is allowed at night after pratikraman and also the next morning.

Notes:

• This is the common Shvetambar lay practice. Some families observe a stricter rule and still wait till Navkarsi even on Tivihar. Always follow the exact wording of the pachkhan you accepted and your guru/paṇḍit’s guidance.
• Digambar practice is generally stricter about night-drinking; many wait till morning.