This is not entirely true. It is perfectly OK to allow your dog to swim in natural water as often as he likes (within reason) but what is not OK is to wash your dog using soaps or shampoos very regularly. Using human shampoos on dogs leads to an imbalance of sweat, and they would be likely to develop dry, itchy skin. Having said that, using mild human shampoos is fine used at a frequency of once per month. However, I would personally recommend dog-specific shampoos and these will have a recommended usage detailed on the bottle, but this would definitely not be more than once every several weeks. In some cases shampoos can be medicated, such as for specific skin-disorders, and the frequency of use for these would depend on the condition.