  Internet of underwater things (IoUT) is a technological revolution that could
mark a new era for scientific, industrial, and military underwater
applications. To mitigate the hostile underwater channel characteristics, this
paper hybridizes underwater acoustic and optical wireless communications to
achieve a ubiquitous control and high-speed low-latency networking performance,
respectively. Since underwater optical wireless communications (UOWC) suffers
from limited range, it requires effective multi-hop routing solutions. In this
regard, we propose a Sector-based Opportunistic Routing (SectOR) protocol.
Unlike the traditional routing (TR) techniques which unicast packets to a
unique relay, opportunistic routing (OR) targets a set of candidate relays by
leveraging the broadcast nature of the UOWC channel. OR improves the packet
delivery ratio as the likelihood of having at least one successful packet
reception is much higher than that in conventional unicast routing. Contingent
upon the performance characterization of a single-hop link, we obtain a variety
of local and global metrics to evaluate the fitness of a candidate set (CS) and
prioritize the members of a CS. Since rate-error and range-beamwidth tradeoffs
yield different candidate set diversities, we develop a candidate filtering and
searching algorithm to find the optimal sector-shaped coverage region by
scanning the feasible search space. Moreover, a hybrid acoustic/optic
coordination mechanism is considered to avoid duplicate transmission of the
relays. Numerical results show that SectOR protocol can perform even better
than an optimal unicast routing protocol in well-connected UOWNs.
