  Underwater sensor networks (UWSNs) are of interest to gather data from
underwater sensor nodes (SNs) and deliver information to a terrestrial access
point (AP) in the uplink transmission, and transfer data from the AP to the SNs
in the downlink transmission. In this paper, we investigate a triple-hop UWSN
in which autonomous underwater vehicle (AUV) and unmanned aerial vehicle (UAV)
relays enable end-to-end communications between the SNs and the AP. It is
assumed that the SN--AUV, AUV--UAV, and UAV--AP links are deployed by
underwater optical communication (UWOC), free-space optic (FSO), and radio
frequency (RF) technologies, respectively. Two scenarios are proposed for the
FSO uplink and downlink transmissions between the AUV and the UAV, subject to
water-to-air and air-to-water interface impacts; direct transmission scenario
(DTS) and retro-reflection scenario (RRS). After providing the channel models
and their statistics, the UWSN's outage probability and average bit error rate
(BER) are computed. Besides, a tracking procedure is proposed to set up
reliable and stable AUV--UAV FSO communications. Through numerical results, it
is concluded that the RSS scheme outperforms the DTS one with about 200% (32%)
and 80% (17%) better outage probability (average BER) in the uplink and
downlink, respectively. It is also shown that the tracking procedure provides
on average 480% and 170% improvements in the network's outage probability and
average BER, respectively, compared to poorly aligned FSO conditions. The
results are verified by applying Monte-Carlo simulations.
