  Water covers 71% of the Earth's surface, where the steady increase in oceanic
activities has promoted the need for reliable maritime communication
technologies. The existing maritime communication systems involve terrestrial,
aerial, and satellite networks. This paper presents a holistic overview of the
different forms of maritime communications and provides the latest advances in
various marine technologies. The paper first introduces the different
techniques used for maritime communications over the RF and optical bands.
Then, we present the channel models for RF and optical bands, modulation and
coding schemes, coverage and capacity, and radio resource management in
maritime communications. After that, the paper presents some emerging use cases
of maritime networks, such as the Internet of Ships (IoS) and the
ship-to-underwater Internet of things (IoT). Finally, we highlight a few
exciting open challenges and identify a set of future research directions for
maritime communication, including bringing broadband connectivity to the deep
sea, using THz and visible light signals for on-board applications, and
data-driven modeling for radio and optical marine propagation.
