Optical communication has been employed in multitudinous applications, including terrestrial, submarine, inter-satellite, and space communications. It enables optical fiber-based communication networks that vastly reshape modern lives through high-speed worldwide Internet connections. With intensified activities such as undersea resource exploration, ecosystem monitoring, and recreation, underwater is an exciting new arena for optical wireless communication (OWC). The recent progress of underwater and water-air OWC systems and devices will be reviewed. Channel characterization, communication system design, and performance investigations will be given. Critical limitations and effective mitigation methods to realize robust underwater and water-air OWC systems will be discussed. In particular, I will fixate on the recently proposed bubble, wave, and turbulence mitigation methods that leverage spatial diversity, advanced signal processing, and beam control techniques. Capacity enhancement for underwater and water-air OWC systems via wavelength division multiplexing will be presented. Finally, other applications for underwater research will be discussed.