This achievement is obtainable just as you reach the rural area and eventually fill the truck with gas in order to break through the gate/bars.
Once you pass through the gate you will come across this small body of water and for the achievement you will have to make it across by hopping on the stones while never touching the water. The best part about this achievement is that it does not force you to reload the last checkpoint if you touch the water. If your feet touch the water, simply turn around, climb back up and try again.
Tip: You will need to hold the left stick down to activate your sprint but you don't really need a running start to make it across from the 2nd stone to the other part of land. Line yourself up, hold the left stick in and simultaneously move forward and press jump.
You will know you did it without touching the water when Brady says "You cant trust the water these days."