  We investigate the optical absorption and scattering properties of underwater
media pertinent to our underwater free space quantum key distribution (QKD)
channel model. With the vector radiative transfer theory and Monte Carlo
method, we obtain the attenuation of photons, the fidelity of the scattered
photons, the quantum bit error rate and the sifted key generation rate of
underwater quantum communication. It can be observed from our simulations that
the most secure single photon underwater free space QKD is feasible in the
clearest ocean water.
