Once again, the author emphasized on water’s symbolism has been lost. In the Fisher King tale, a foot washing ceremony preceded the restoration of the King. However, in the poem, they used soda water to wash their feet instead of water.
In addition, in Christianity, water is used for baptize. It is a religious ritual, where people use clean & clear water to wash away their origin sins. But soda water is unclean water, therefore, people do not wash away their sins. Perhaps they even add more sins - commit more immoral things than they suppose to.