  In the sixth-generation (6G) Internet of Things (IoT) networks, the use of
UAV-mounted base stations and reconfigurable intelligent surfaces (RIS) has
been considered to enhance coverage, flexibility, and security in
non-terrestrial networks (NTNs). In addition to aerial networks enabled by NTN
technologies, the integration of underwater networks with 6G IoT can be
considered one of the most innovative challenges in future IoT. Along with such
trends in IoT, this study investigates the secrecy performance of IoT networks
that integrate radio frequency (RF) UAV-based NTNs and underwater optical
wireless communication (UOWC) links with an RIS. Considering three potential
eavesdropping scenarios (RF signal, UOWC signal, and both), we derive
closed-form expressions for secrecy performance metrics, including average
secrecy capacity, secrecy outage probability, probability of strictly positive
secrecy capacity, and effective secrecy throughput. Extensive numerical
analyses and Monte Carlo simulations elucidate the impact of system parameters
such as fading severity, the number of RIS reflecting elements, underwater
turbulence, pointing errors, and detection techniques on system security. The
findings offer comprehensive design guidelines for developing such a network
aiming to enhance secrecy performance and ensure secure communication in
diverse and challenging environments.
