Yes it is true that improvement in water and sanitation can control many diseases. If the water which we use for all our propose is contaminated with come contaminators then the we the ones who are using that water will be at a loss because we use that water and we may consume that contaminator and get ill. So we need to make sure that water which we use is clean and that don't contain any harmful contaminator.

example :
1. If suppose the water which we use for all our purposes contain more amount of floride in it then causes fluorosis