HighlightsTourism development on the south coast of Rio de Janeiro;Water quality impacts resulted from human presence;We selected 20 water sampling stations and evaluated the seasonal water quality variation;Water quality changes between seasons became clear;The Water Quality Index (WQI) was very useful for the classification of the waters monitored. PurposeThe main objective of the present paper is to detect the tourism influence in the water quality of the municipality of Paraty and surrounding water bodies, analyze individual parameters and use the WQI.Design/methodology/approachIn the present paper a simple management tool was applied in order to prevent future tourism impacts in coastal areas.FindingsAlthough considered in its natural state, the area already showed impact signals.Research limitations/implicationsThe analysis need to be more enhanced for finding the spatiotemporal evolution in the estuarine environment.Practical implicationsThe paper provides a starting point for minimizing the negative impacts of tourism.Originality/valueThe present paper contributes so that tourism may become a positive factor of local development.