so putting water decreases flow if too much is added. This is because you’re also diluting the lubricants in the ink. Adding water does decrease the concentration of dye, but that’s only one component of flow issues.
If you want to dilute the dye but don’t want to dilute the rest of the ink, what you need is a dilution solution (also known as ink diluent). the ones I know of are the De Atramentis Dilution Solution and the Platinum one that is in the Platinum Mixable ink mixing kit.