Short answer: in strict Jain fasts, you should avoid sweetened water. Water itself is allowed or restricted depending on the exact vow (Tivihar upvas allows boiled water only, within daylight; complete upvas usually forbids water). Sugar or any sweetener with water is generally not part of the strict fasting rules and is avoided, but practices can vary by sect and local guru. When in doubt, follow the guidance of your local Jain sangh or teacher.

You can read more here and here and see related discussions about fasting rules during Paryushan.