In Jain dharma, if water is taken during a fast and it is against the vow you are observing, then one should do pratikraman as soon as possible and repent sincerely for the lapse.

A simple understanding is:

• Immediately stop and become aware
• Do heartfelt pratikraman / alochana
• Resolve not to repeat it
• If possible, do the formal pratikraman at the earliest suitable time

If the water was taken accidentally, unknowingly, or due to a genuine mistake, the inner feeling of repentance is still important. In Jain practice, bhav matters greatly.

So, in short: yes, pratikraman should be done without delay, as soon as you realize it.