Does drinking too much fluid/ water put strain on kidneys, making them work harder? in general what to do keep kidneys healthy?

Not usually.: A normal healthy person without any underlying kidney disease does a pretty good job of balancing all the water and electrolytes going through the body. Uncommonly, a long distance runner can drink too much water to the point of dangerously low sodium. Eating well and exercising - anything to help prevent diabetes and high blood pressure - will help keep the kidneys healthy!