“Does drinking water lower blood sugar?”

not completely but keeps you hydrated

Generally, drinking water does not lower your blood sugar, although having enough water is certainly vital to health and the good function of various body systems. When sugar readings in the blood are very high (glucose readings, glucose is the form of sugar found in the bloodstream) IV fluids are given as one strategy to help with lowering the blood sugar, but that is a rather different situation and process. It is also only one part of treating a very high glucose.

It depends of what type of diabetes mellitus.