Except in extreme situations (as in, in a hospital on IV fluids.. In 120 degree heat with a dehydrated mum) infants shouldn't be given water on top of breast milk or formula. It upsets their electrolyte balance and can cause seizures, uncontrollabe fever & febrile seizures, and mess with their organs.
To know... There's a LOT of chemical communicatin between baby's mouth and mom's breast. When the chemicals are demanding higher mineral content, far content, sugar content, WATER content, our bodies adjust milk components. Part of Failure to Thrive CAN be a miscommunication between chemicals, so breast milk is not only not best, but often worst. HOWEVER most of the time, the communication between baby's body and mom's body is dead on. Adding things (like water or solids) makes the communication slow down get off a bit. Just like giving formula slows down moms supply increasing.
If you're exclusively breast feeding, and all is good, no reason to make things harder by throwing a wrench into a working system.