It is said that we shouldn't give water to the baby before 6 months, so can we give jeewanjal or oral rehydration salt to the infant if they suffer from diarrhea?

Yes, ORS or jeewanjal during diarrhea could be life saving for everyone including infants under 6 months old. Exclusive breastfeeding should be continued during this period to prevent dehydration and malnutrition. WHO and UNICEF jointly recommends use of ORS and zinc (10 mg perday for infants under six months old for 10-14 days), to prevent the severity of the episodes and prevent further occurrences as well. (Source : UNICEF, WHO)