A personal water safety device includes at least two sonar receivers, at least one water sensing device, and an alarm apparatus. The at least one water sensing device communicates with each of the at least two sonar receivers via sound waves. The alarm apparatus wirelessly communicates with each of the at least two sonar receivers. Each water sensing device is worn by a swimmer and is triggered to measure an elapsed time when the swimmer submerges in water, and transmits the measured time and a corresponding identification number to the at least two sonar receivers via the sound waves. The alarm apparatus receives the measured time and the corresponding identification number transmitted from each of the at least two sonar receivers, and generates an alarm when the measured time of one of the at least water sensing device exceeds a predetermined time limit.