Drowning is the second leading cause of unintentional injury death for children aged 1–14 years. This paper presents the development of an early drowning detection system based on the Bluetooth Low Energy technology and the received signal strength-based localization method that allows lifeguards to monitor the position of swimmers in real time. Swimmers wearing Bluetooth tags are monitored by four Bluetooth beacon scanners installed in the swimming area. The wearable Bluetooth tags periodically send beacon signals to the Bluetooth beacon scanners. Locations of swimmers are estimated using an improved RSSI-based algorithm. The radio links between Bluetooth tag and beacon scanners will be interrupted when the victim is submerged under water for certain duration. Sirens placed in the monitored area are then triggered to alert lifeguards about drowning accident. The proposed system helps to narrow down the search area and reduce the rescue time when drowning occurs.