It is true that tea and coffee are dieuretics, but it is a falacy that they cause water loss. I asked my Doc about just this subject when I was first doing the diet and he said that the amount of water in a cup of tea or coffee is far greater than any water loss caused by the drinking - so the net effect is a gain in water intake.
That said, I probably drink 2 or 3 cups a day, so my water intake from that pretty much makes no difference, so I never count it.