If you don't have hard water, maybe. Not here. Soap scum on the tub, soap doesn't remove dirt, just sort of locks it on. Which does not matter as much on skin but is disastrous for hair. I have known a couple people who had to cut their hair off after trying to wash it with soap.

If I was to the point where I could only afford one, I'd get shampoo. If that was impossible for awhile, water only is the way I would go. It works ok, most stuff rinses off.