This paper presents a bearings-only tracking (BOT) scheme based on distributed floating beacons in underwater sensor networks (UWSNs). Conventional BOT systems extract target-originated bearings in noisy measurements, locate and track targets based on the bearings in real time. We propose a tracking scheme by modifying BOT systems for UWSNs where a multidimensional assignment (MDA) method is used to extract target-originated bearings and to associate them with corresponding targets. In addition, a threshold-based decision in extracting bearings in MDA and a local Kalman filter is proposed to keep tracking information at a floating beacon even if a target is undetected. Simulation results show that the proposed scheme has advantage of tracking accuracy over recently known work under low detection probability.