Water is healthy for your kidneys and water does help to flush your kidneys out.
But yes it is possible to drink too much water and cause harm to your kidneys which can do more harm to your kidneys than it would help.
So do not drink too much water but drink enough water to keep your kidneys and body healthy.
It's recommended to drink at least 8 glasses of water per day but you could drink 10 or 12 glasses of water per day if you want too.
Just avoid drinking any more than 12 glasses of water per day and your kidneys should be just fine.
However it's unlikely even if you drank more than 12 glasses of water in a day that it would do harm to your kidneys and you'd probably have to drink around 2 or 3 gallons of water per day to damage your kidneys.
When you drink too much water then your kidneys cannot process that water fast enough which then leads to kidney damage.
When your kidneys cannot excrete the excess water then the sodium content of your blood becomes diluted which then leads to (hyponatremia) which can be life threatening and requires medical attention to save your life.