Monitoring water levels and surface areas at reservoirs is essential for managing drought and flood risks, but it can be expensive and labor-intensive. In South Korea, over 3,000 agricultural reservoirs require about 50 million USD annually for maintenance. A new satellite mission from NASA, called SWOT (Surface Water and Ocean Topography), launched in 2022, offers a way to measure water levels and surface areas from space for large reservoirs. However, its data need to be checked for accuracy against ground-based measurements. This study compares SWOT data with on-site measurements from 32 dams and 1,668 reservoirs in South Korea. It also includes a detailed local survey using uncrewed boats and drones equipped with advanced instruments to measure water levels and areas. The comparison shows that SWOT's water level estimates are fairly accurate, with a median difference of 0.75 meters nationwide and only 0.10 meters at the local site. However, its surface area measurements vary more, mainly due to irregular sampling times.