I brush my dogs' (Schnauzers) teeth...though not as often as I should. I focus primarily on the outside of the teeth, I've read somewhere that the inside don't need brushing...not sure if that's true or not, but brushing the inside is next to impossible anyway. The back teeth are tricky. For that I bought an easybrush chew (see link below). It is a hard rubber bone-shaped chew with bristles in the ends. You put a drop of toothpaste into the bristles and let them chew on it for a couple of minutes. They mostly chew on it with their back teeth, which is what I use it for.
http://www.easybrush.com/
Edited to add: You mentioned water...you don't need to use water or rinse their mouth. In fact, you want the toothpaste to stay on their teeth for as long as possible as the enzymes continue to work even when you've finished brushing.