Underwater wireless optical communication (UWOC) has attracted much attention recently, thanks to its high transmission speed. In many UWOC applications, the transmitter is based on light emitting diodes (LEDs). This is because it has up to a 120° divergence angle, which is helpful with establishing the communication link more easily. However, the light field of such a transmitter is affected by watertight encapsulation in practice. In this paper, we establish a theoretical model based on the Monte Carlo method to study this influence when the watertight optical window is based on plain glass. The results show that both the divergence angle and transmittance become smaller because of the optical window. In order to reduce this influence, the illumination surface, distance from the light source to the optical window, and thickness of the optical window should be smaller, while the optical window surface should be larger. In the ideal situation, the largest divergence angle is 94.4°, which is defined with full width at half-maximum (FWHM). The maximum transmittance is related to the optical materials of the window, while it is 93.54% for quartz glass. When light is modulated with on-off keying (OOK) format, if the signal-to-noise ratio (SNR) is larger than 40 at 0° radiation angle, the UWOC system could work when the angle is up to ±48.7∘. While the SNR reduces, the workable angle range decreases. The simulation model presented in this paper is convenient to analyze the influence of a watertight optical window with plain glass. The simulation results presented in this paper are useful to the UWOC transmitter design.