I present in this thesis the results of my research in the field of underwater optical communication system (UOCS). Recently, underwater free-space optical (FSO) communication has been one of the major interesting research subjects due to the high demand for underwater activities that require high-bandwidth and flexible solutions. An optical communication system consists of three main blocks: transmitter, medium and receiver. Therefore, I provide an introduction to the topic, outlining the physical reasons and the engineering challenges that are behind the following sections. Among others, the optical properties of ocean water, the optimum wavelength selection and a description of the main system components are provided so that the system link budget can be subsequently optimised. Various modulation schemes adopted in free-space optics communication are evaluated and compared with a focus on the most power-efficient modulation format that offers successful and reliable data transmission. The system performances are numerically investigated using statistical analysis techniques over a typical range of achievable SNR in an underwater scenario. The characterisation and the data performance of a commercial GaN-based laser diode operating at 450 nm are presented. The detrimental impact of the solar background power on the system performance and the strategy to minimise its contribution is also discussed for a conventional Silicon PIN detector and the novel Silicon PhotoMultiplier (SiPM) technology. The design trade-off between the performance improvement given by an optical narrow bandpass filter matching a Fraunhofer line and the field-of-view of the receiver is also presented. A flexible MATLAB model has been developed to simulate a range of different scenarios and evaluate the system performances in different situations and the necessary design trade-off. A novel free-space waveguiding method is also presented for laser-based underwater communication systems. The proposed Underwater Wireless Acousto-Optic Waveguide (UWAOW) generates a localised modification of the refractive index of seawater in response to an acoustic field. Two geometries and their modelling are provided in order to take advantage of the proposed technique. These results show the importance of this emerging, challenging and fascinating contemporary research field.