The Coronavirus Disease (COVID-19) pandemic induced paradoxical changes in the coastal environment of South Korea. In this study, we found that following the outbreak, a significant reduction in tourist activity led to a 69.2% decrease in the amount of coastal waste. Conversely, an increased load of terrigenous organic matter (OM) from altered human activities resulted in higher concentrations of OM and chlorophyll-a in coastal waters. These changes led to contrasting effects on dissolved oxygen (DO) levels. To quantify these impacts, we employed satellite sensor data coupled with a machine learning model to spatially predict DO dynamics. In the deep offshore waters, enhanced photosynthesis from the phytoplankton bloom was predicted to have increased surface DO concentrations. However, in the shallow, semi-enclosed coastal waters, the increased OM load led to a decrease in DO concentration throughout the water column owing to heightened oxygen consumption. These findings underscore the complex and spatially varied responses of coastal ecosystems to widespread shifts in human behavior, highlighting the utility of advanced remote sensor technologies in coastal monitoring.