A: An underwater 137 Cs (662 keV) and 131 I (364 keV) radioisotopes monitoring system has been developed using a 3 × 3 NaI(Tl) detector. To estimate the minimum detectable activity and Bq/L conversion factor, an experiment was first conducted with 68 Ga (511 keV) in a water tank. For energy spectrum comparison, Monte Carlo simulations were performed for 68 Ga, 137 Cs, and 131 I to determine the conversion factor. The minimum detectable 137 Cs and 131 I activities for the system are 0.77 and 0.89 Bq/L, respectively.