If you look carefully as you approach you can see G-Man looking at you before he disappears again.
You'll see a large red barn up ahead - drive up to it and stop at the steps at the base. Head back down, get back in your boat and carry on outside (drive into the wooden planks to smash them). Get off here and you'll find a ladder that you can climb to reach two boxes of pistol ammo, yielding a total of 40 bullets. At the bottom of the drop, a short way along past the barrels, you'll find a lambda symbol sprayed on a wall to the right of the river.
Continue to follow the river, and you'll go over an edge and fall to some clean water closer to the outside of the city.
