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In this paper, we propose a floating beacon-assisted 3-D localization for variable sound speed in underwater sensor networks (FBL-VSS). Most underwater localization schemes have assumed that the sound speed is constant under water for simplicity; however, it is actually variable depending on depth and seasonal variation. Taking into account various sound speed, FBL-VSS utilizes novel beacons moored to the sea floor. Each beacon has a transmitter floating on the sea surface and a receiver placed on the sea floor, in order to obtain the average of the actual sound speed from the sea surface to the sea floor by measuring the time of flight (TOF) of broadcasted messages. For self-location, FBL-VSS uses multidimensional scaling techniques with distances computed from the average of the actual sound speed. Simulation result shows that our proposed scheme has stable location accuracy for variable sound speed profile under water.