  Quantum key distribution (QKD) has undergone significant development in
recent decades, particularly with respect to free-space (air) and optical fiber
channels. Here, we report the first proof-of-principle experiment for the BB84
protocol QKD over a water channel. Firstly, we demonstrate again the
polarization preservation properties of the water channel in optical
transmission according to the measured Mueller matrix, which is close to the
unit matrix. The reason for the polarization preservation, revealed by Monte
Carlo simulation, is that almost all the received photons are unscattered.Then,
we performed the first polarization encoding BB84 protocol QKD over a 2.37m
water channel. The results show that QKD can be performed with a low quantum
bit error rate (QBER), less than 3.5%, with different attenuation coefficients.
