There should be no need for water when brushing your teeth!
I've heard that you should just apply toothpaste to the brush, brush then spit. If you wet your brush, or rinse your mouth with water you are diluting the fluoride content in the toothpaste and hence it is not as effective.
I've also heard that you should use mouthwash
before you brush your teeth rather than after. Mouthwash usually has fluoride in the region of 250ppm, whereas toothpaste is usually 1000+ ppm. If you rinse with mouthwash you will not have as high a concentration of fluoride in your mouth.
This is all from a dentist
.
Well, I suppose you need water to rinse your toothbrush after you've used it!