  We analyze the secrecy performance of a two-hop mixed radio frequency
(RF)/underwater wireless optical communication (UWOC) system using a
decode-and-forward (DF) relay. All RF and UWOC links are modeled by the
$\alpha-\mu$ and exponential-generalized Gamma distributions, respectively. We
first derive the expressions of the secrecy outage probability (SOP) in exact
closed-form, which are subsequently used to derive asymptotic expressions at
high SNR that only includes simple functions for further insight. Moreover,
based on the asymptotic expression, we can determine the optimal transmit power
for a wide variety of RF and UWOC channel conditions. All analyses are
validated using Monte Carlo simulation.
