High-speed visible light communications (VLC) has been identified at an essential part of communication technology for 5G network. VLC offers the unique advantages of unregulated and secure channels, free of EM interference. Compared with the LED-based VLC transmitter, laser-based photonic systems are promising for compact, droop-free, and high-speed white lighting and VLC applications, ideal for ultra-fast 5G network and beyond. Besides the potential for achieving high data rate free-space communication links, i.e. the Li-Fi network, laser-based VLC technology can also enable underwater wireless optical communications (UWOC) for many important applications. In this paper, the recent research progress and highlights in the fields of laser-based VLC and UWOC have been reviewed with a focused discussion on the performance of various light sources, including the modulation characteristics of GaNbased edge emitting laser diodes (EELDs), superluminescent diodes (SLDs) and vertical-cavity surface-emitting lasers (VCSELs). Apart from the utilization of discrete components for building transceiver in VLC systems, the development of III-nitride laser-based photonic integration has been featured. Such on-chip integration offers many advantages, including having a small-footprint, high-speed, and low power consumption. Finally, we discuss the considerations of wavelength selection for various VLC and UWOC applications. Comparison of infrared (IR) and visible lasers for channels with high turbulence and the study of ultraviolet (UV) and visible lasers for non-line-of-sight (NLOS) communications are presented.