The idea of using free space optical communications underwater has developed recently to investigate the oceans for scientific and financial reasons. Underwater wireless optical communications (UWOC) is a newly emerged technology which uses the optical carriers to transmit information underwater. In comparison to acoustic waves that were used previously in underwater wireless communications, employing optical waves increases the speed and bandwidth of data transmission, significantly. However, using optical waves is limited to short distances due to high water absorption in optical frequencies. Fortunately, the lowest absorption of water in green-blue wavelengths, makes the visible light as a good option to provide high bandwidth communications in tens of meter link span. This short distance is used for applications such as maintaining oil equipment, monitoring ports and connections of submarines to lands in wireless sensor networks. Due to the unique features of water, channel models of free space optical communications are not suitable for UWOC. To provide acceptable data rate and bit error rate for underwater optical communications, it is necessary to investigate the effective parameters on light propagation in the water. In this research, the physical model of UWOC channel including absorption and scattering phenomena based on the Monte Carlo approach is studied. The effects of absorption and scattering on different aspects of communications system performance such as power loss, time dispersion (channel bandwidth), transmitter and receiver misalignment and angular spreading (angle of arrival distribution) in clear and harbor water are investigated. In addition to underwater channel characteristics, the parameters of optical communications system affect the system performance, as well. Among different system parameters, the effects of divergence angle of Gaussian beam transmitter and its limitation cases including plane and spherical beams, size of the aperture, field of view and sensitivity of detector on the power loss, channel bandwidth and receiver offset are investigated. In addition to absorption and scattering, variations of water refractive index cause turbulence underwater. Despite the existing physical models including absorption and scattering in previous researches, a physical turbulence model for the applications of UWOC is not presented yet. In this research, a turbulence model based on the refractive index variations in a horizontal UWOC link with the Monte Carlo approach is presented which is far less computationally intensive than approaches based on computational fluid dynamics. Furthermore, it is more simple and flexible for UWOC applications. Model results show that the lognormal distribution is fitted well with the probability density function of the received light intensity fluctuations in weak, moderate and strong turbulence regimes, while in the saturation regime it follows the negative exponential distribution. Variations of scintillation index with the channel parameters including link span and refractive index variation and with the system parameters including divergence angle of Gaussian beam transmitter, aperture size and field of view of the detector are investigated. Moreover, the proposed model predicts the turbulence induced power loss especially in longer link span and higher refractive index variations. To verify the proposed turbulence model, experimental tests are achieved. A new practical method is introduced to produce turbulence. Weak turbulence is investigated up to 12 meter link span. The link span is one of the main factor defining turbulence strength and its effect on the probability density function of the received light intensity and the scintillation index is investigated in the experiments. The results of the experiments and the proposed model are in good agreement. Keywords: Underwater wireless optical communications, Channel modeling, Absorption, Multiple scattering, Turbulence, Loss, Time dispersion, Misalignment, Angular spreading, Transmitter beam divergence angle, Probability density function, Scintillation index, Aperture size, Field of view