In this work, an underwater visible light communications (VLC) vertical link performance is evaluated. Underwater environment is known of its inhomogeneous nature versus depth. A mathematical model for the received power (Pr) is derived and bit error rates (BER) are computed under different underwater conditions. A numerical example is given for illustration of the deduced model. Our results suggest that an optimum transmitter-receiver separation exists, where BER is minimum at a certain transmission orientation.