Funny, a friend and I were talking today and she said her friend hasn't used anything but water in over 30 years and the dentist is always super impressed with how healthy her gums are. No cavities either of course. Maybe I should just brush with nothing!
I've heard from my dentist (and also, recently, a dentist on the radio) that toothpaste is just not necessary for healthy teeth and gums (why is spellcheck telling me that "gums" is spelled incorrectly? Bizarre.) They say that it's good for the minty fresh taste/feeling, but the real goal in brushing your teeth is removing the plaque that's sitting there, which a brush and water are perfectly capable of achieving.
I'm sure that's true, but I'm sticking with my Crest!