Light emitting diode (LED) based underwater optical wireless communications (UOWC) aims to provide high-speed data rates in the underwater environment. The field of UOWC has been receiving greater attention in recent years as a viable complementary technology to meet the data rate demands for ocean exploration. It offers a competitive advantage of high-speed, low delay and low cost than radio frequency (RF) and underwater acoustic communications (UAC) technologies in the short-distance underwater link. However, there are still research challenges that need to be overcome, one of the primary challenges is the small modulation bandwidth that limits the transmission rate. This thesis develops a good understanding of the LED-based UOWC applied spectral efficiency modulation schemes as well as the inherent optical properties (IOPs) in underwater conditions. The thesis begins with an introduction of background for underwater wireless communications (UWC). Comparing the merits of acoustic waves, RF and optical waves. Optical wireless communications (OWC) is superior at high data rates with low latency in medium underwater links (up to 10s of meters), which attracts the researchers’ attention and interest. However, UOWC still has some challenges as followed: light suffers from severe absorption and scattering, misalignment of optical transceivers, and reliable underwater devices. To begin with, the UOWC system is described in terms of the optical sources and detectors used. Here, LEDs have been found to be more suitable for certain applications in underwater links than laser diode (LD). Additionally, various modulation schemes and equalisers are used in this thesis to overcome the impairments associated with the limited bandwidth of LED and the resulting inter-symbol interference (ISI) that is stated. Next, the interaction of light and water is described as well as the IOPs measurement. Furthermore, considering the measurement of the natural water environments is difficult, the channel model is commonly used to evaluate the performance of the UOWC transmission. An underwater channel model based on double-gamma function (DGF) is used to investigate the performance of UOWC before experiments are carried out. Following these, experimental works related to the LED-based UOWC system setup, design of water conditions, the frequency response of LED-based UOWC link, signal-to-noise-ratio (SNR), and results of data transmission are presented. The impact of turbid water conditions on UOWC is experimentally characterised. The performance of pulse amplitude modulation (PAM), carrierless amplitude and phase modulation (CAP), and quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM) is then compared in the previously characterised UOWC channel. These results will contribute to the further development of the LED - based UOWC in realistic channel conditions. In conclusion, the maximum data rates of different modulation schemes are presented. QAM-OFDM has an overall data rate advantage over CAP and PAM but gradually loses the advantage in turbid water conditions. Using QAM-OFDM with bit and power loading (BPL) resulted in a maximum data rate of 557 Mbps in clear water, but CAP has a maximum data rate of 113 Mbps in the most severe turbid water condition considered in this study.