An old man at the shower room convinces Rinko that the water at the beach is unclean and letting it enter her body is unsafe, the only way to clean it out is with organic soap.
Having her common sense completely altered, she happily accepts his bar of soap.
Rinko visiting clearly suspicious areas like under the bridge always seemed like subconscious alteration of common sense to me.
I want to explore that more fully in the remake. This is the third stage of the new expanded shower event.