Your pediatrician is just dead wrong.

An exclusively breastfed baby CANNOT become "overweight." It's just impossible.
http://www.kellymom.com/babyconcerns...t-toomuch.html
It's very normal for BF'd babies to shoot up in the charts in the first six months, and then plateau out between six months and a year. It's textbook, in fact. Formula-fed babies, on the other hand, tend to continue to grow at the same rates for the second half of the first year, and can be overfed.
My baby was 24 lbs at six months, in the 90s as far as percentiles for both weight and height, but my doctor wasn't concerned. He is still a big boy, but his weight gain slowed down considerably, and at almost three years, is just shy of 35 lbs (down in the percentiles to the 60-70s). Again, very normal for a breastfed kid. At first they pack on the chub, but once they get moving, they slim down considerably.
Don't limit your baby's breastmilk intake! Don't supplement with water!!!!!! This is terrible, terrible advice that would only apply to a formula fed child. I normally wouldn't be saying this, but get a second opinion, find a new doctor, or just read up on it and blow off your doc. He's just flat out, 100% WRONG about this.