  We demonstrate the underwater quantum key distribution (UWQKD) over a
10.4-meter Jerlov type III seawater channel by building a complete UWQKD system
with all-optical transmission of quantum signals, synchronization signal and
classical communication signal. The wavelength division multiplexing and the
space-time-wavelength filtering technology are applied to ensure that the
optical signals do not interfere with each other. The system is controlled by
FPGA, and can be easily integrated into watertight cabins to perform field
experiment. By using the decoy-state BB84 protocol with polarization encoding,
we obtain a secure key rate of 1.82Kbps and an error rate of 1.55% at the
attenuation of 13.26dB. We prove that the system can tolerate the channel loss
up to 23.7dB, therefore may be used in the 300-meter-long Jerlov type I clean
seawater channel.
