The BIGGEST IMPACT! would be a HEALTHY CITIZEN, an asset to his nation and ultimately a healthy society, leading to the goal of humanitarian of activities going all around the world of any form!
And a suggestion isn't "is" supposed to be "if" here in the question?
The world would be a much safer place. The biggest impact would be the enhancement of human well-being and increased alleviation of poverty. The spread of waterborne diseases will drop, infant mortality will drop and there will be huge savings on health expenditure. Throughout history, we have had numerous conflicts over resources. Would the next major conflict be over Water?
The amount of time and effort lost through illness, collecting and transporting water would unlock so much potential for other activities - particularly education and income generation, particularly for girls and women. Gender parity is generally better in societies where girls and women are no longer slaves to domestic chores, and this has a profound benefit for the whole society.