Anecdotal sources suggest that if you've noticed that you've added too much water to your mix, all you need to do is compensate it with extra dry ingredients. Just add a few tablespoons flour to thicken it, but be cautious until the texture is right.
About a tablespoon per unit of flour you added should do. Make sure to stir after each addition, because too much flour will toughen and dry the brownies. Check the consistency to make sure it's not too runny and then bake them.
If the batter is still watery, your best alternative is probably to start over with a new batch of brownies.
We don't mess around when it comes to brownies. But even if you have the perfect recipe, that doesn't guarantee flawless results. Check out some tips to make perfect brownies every time at Huffingtonpost.com.